Since asking the question I have now read three books by influential pentecostal pioneers on the issue, and they do not agree on the definition of the gift of the word of knowledge.
In the book on Spiritual Gifts, a collection of sermons by Smith Wigglesworth (1859-1947), who received the baptism of the Spirit in 1907, the gift is described as a quickening that gives understanding of the Scriptures:
It is as we feed o the Word and medidate on the message it contains that the
Spirit of God can vitalize what we have received and bring forth through us
the word of knowledge... As all the Scriptures were inspired by God
(2 Tim 3:16) as they came forth from the beginning, and though the same
Spirit they should come forth from us vitalized...
It is unclear if SW think that the essence of the gift is the act of illumination per se, or if it also must include a speaking forth of the knowledge received to others. The passage in the book is very short and neither exegetically or systematically complete.
A view more in line with the current understanding within the charismatic movement is given in the book Questions & answers on spiritual gifts by Howard Carter (1891-1971). The book consist of stenographic notes from a Q and A session at Hampstead Bible College in London. Carter claims that he received his insights on the gifts during imprisonment as a conscientious objector during world war one.
Carter says on page 43:
The word of knowledge is a participation, to some infinitesimal degree, in
the omniscience of God. If the Lord who knows all things, is pleased to
reveal to us by His Spirit any fragment of His unlimited knowledge, then we
can claim to have this manifestation of the Spirit, which is designated
"the word of knowledge".
Furthermore, Carter says, that the fragment may be received by the mind as a thought, but it is not created by the human, only received (page 53). The process is completely supernatural. It is not the by-product of diligent study, nor is it in any way connected to education (page 38, 46, 49-50. (Carter does not despise diligent study, but it is unrelated to the definition of the gift.)
The scope of the knowledge includes everything that God knows, which may be worldly facts as when Jesus saw Natanael sitting under a fig tree (John 1:48) or when Ananias was told about the name of the street where Paul was praying after his encounter with Jesus on the road to Damascus (Acts 9:11) (Carter:45). But what makes this a very eminent gift is that God has used it to reveal His very own nature to the authors of the Bible (Carter:43-44). Carter is, however, clear that no new insights can be given that conflicts with Scripture or appends new doctrines to it (page 23-24).
Revelation about the purpose and plans of God, and of the future are not words of knowledge, but words of wisdom (Carter:17ff).
Carter is very clear that we should not talk about the gift of knowledge or the gift of wisdom, but the gift of the word of knowledge and the word of wisdom (page 20-21), as we are all expected to grow in wisdom and knowledge through a plethora of means.
A very different view is offered by Donald Gee in the book Concerning Spiritual Gifts. The book was originally published 1947, but builds upon earlier works published 10 and 20 years earlier (page 133), and Gee says that his views on this subject "have undergone practically no change".
According to Gee, the word of knowledge is a teaching gift in the church (page 134. However, before he arrives at that conclusion, he has a few words of warning:
The Scriptures provide for the task [of defining the gift] no material
that is avowedly and unmistakably a manifestation of the word of knowledge...
Assumption is not proof... We ought carefully to avoid dogmatizing on this
subject. (page 41)
Having acknowledged the types of supernatural revelations that Carter speaks of in a way that affirms that Carters view indeed was held by many, Gee proceeds to call such revelation prophecy, neither denying its existence or its value (page 42). But when it comes to the definition of the gift, he still begs to differ.
For Gee the setting of 1 Cor 12 is of utmost importance, it is in the gathered Church that the gifts are to be used, whereas most of of Carters examples are from happenings in individual lives. In agreement with Carter, Gee claims that the gift is completely supernatural (page 44f), but it is not "spectacular" (page 45-46).
There come times when the Spirit of revelation is so operating through a
teacher exercising an anointed ministry that we become conscious of an
illumination transcending all natural ability either to gain or to impart
(knowledge). It is in such hours that the sheep hear the voice of their good
Shepherd speaking through human lips... We know it because our hearts burn
within us as surely as theirs did upon the Emmaus road when Christ "expounded
unto them in all the scriptures the things concerning himself". By the gift
of of the Spirit that Voice still expounds the Scriptures on the sweetest of
all themes - himself (Gee:46-47)
In the book edition from 1947, Gee has added a whole chapter that was not part of the edition from 10 years before, where he gives a systematic argument in favor of his interpretation. However, those arguments are outside the scope of this question on StackExchange.
In summary we see that although Gee argues strongly for his view and he at least is in some agreement with an influential preacher (although he was not a "teacher") like Wigglesworth, he pleads his case as if he perceives himself to be in a minority view. At least that is how I judge his tone. Thus it seems that Carter's view was more widespread within early pentecostalism. But it did not enjoy complete dominance.
Later teachings (2nd generation pentecostalism/charismatics/neo-pentecostals) are mostly using definitions similar to carter, but they are, according to the teaching I've read and heard, very reluctant to include revelation about the nature of God, and restrict the gift to worldly knowledge.
According to the view within the pentecostal movement. I am aware that this view is not shared by all Christians, but it is a significant fact to date the involvement of SW in pentecostalism.
Wigglesworth, S. (1998). Smith Wigglesworth on spiritual gifts. New Kensignton, PA: Whitaker House. Pages 76-77
Carter, H. (1976). Questions & answers on spiritual gifts. Tulsa, OK: Harrison House.
Off topic fact: Pacifism was very common among the early pentecostals.
Gee, D. (1980). Concerning spiritual gifts. Sprinfield, Mo: Radiant Books.