I read from a little book called the Manual of Catholic Devotions (by Regina Press, with an imprimatur dated 24 July 1963) that the rules for fasting and abstinence in the United States at that time were:
Abstinence alone is prescribed every Friday, unless a holiday of obligation falls thereon.
Fasting and Abstinence are prescribed in the United States on the Wednesdays and Fridays of Lent and Holy Saturday forenoon (on all other days of Lent fasting alone is prescribed and meat is allowed once a day), the Ember Days, viz., the Wednesday, Friday, and Saturday following the First Sunday of Lent, Pentecost or Whitsunday, the 14th of September, and the Third Sunday of Advent; the Vigils of Pentecost, All Saints, Immaculate Conception, and Christmas.
There is no fast or abstinence if a Vigil falls on a Sunday. Whenever meat is permitted, fish may be taken at the same meal.
The same book defines fasting and abstinence as follows:
Abstinence forbids the use of flesh-meat and of the juice thereof (soup, etc.). Eggs, cheese, butter and seasonings of food are permitted. The Law of Fasting forbids more than one full meal a day, which must not be taken before noon.
Based on the description there doesn't seem to be much difference between the Ember Days in Lent and the other days in Lent: both fasting and abstinence are required on Wednesdays and Fridays of Lent, so the Lenten Ember Wednesday and Friday appear to have the same rules as any other Wednesday or Friday in Lent. Therefore only Ember Saturday appears to have an additional restriction of abstinence compared to other days in Lent.
Is this correct? If so, is the similarity between the Lenten Ember Days and the other days of Lent at this time (early 1960s) due to the relaxation of fasting/abstinence rules over time, or were they always so similar?