Here is a very good website which shows just about all of the RCA 2 track tapes that RCA released including pictures of the tapes.
There are three different files, with the mono tapes (usually using the left and right channels with separate information), the first stereo tapes from 1954 to 1957, and a second set of stereo tapes from 1957-1960. RCA ended producing 2 track tapes in 1960, so any album, like the Sonny Rollins The Bridge (recorded in 1962) never was released on 2 track by RCA. The majority of the 2 track tapes were issued before the summer of 1958, when RCA released their first stereo records. So for those tapes, it seems impossible that the vinyl lacquers were cut before the tapes were made. If anyone knows if RCA cut lacquers say in 1954 when the first stereo tapes were recorded and kept them until 1958 when they released the first stereo records, please let me know. That seems impossible to me.
The earliest stereo tapes from RCA are all classical, with pop albums starting to appear in 1957. By the end of the 2 track tapes starting in 1959, almost all the titles were pop. Since I only have classical RCA 2 track tapes, all of them appear to have been recorded prior to August, 1958 when RCA began releasing stereo records. So my assumption is that these tapes were duplicated before any stereo lacquers were cut from the original tapes. Any high frequency loss from the loss of freshness of the original tapes should be more pronounced in the vinyl than the tapes.
Pacha, your 300 RCA early 2 track tapes looks like it could well be very close to a complete collection - there are only about that many tapes pictured on the website that I have listed above, including all the different variants in covers for the tapes. You have a very valuable collection!
Larry