On Thursday, November 15, 2018 at 5:00:12 PM UTC-8, wrote:
Back when the rains came & the earth cooled:

400-series academic courses were associated with a cadet's freshman ("4-degree") year.
AM-451 was typically taken the summer break before a cadet's began his sophomore year.

The USAF ACADEMY SOARING PROGRAM SYLLABUS OF INSTRUCTION FOR AIRMANSHIP 451, BASIC COURSE GLIDER, Dated APRIL 1980, states, "This syllabus outlines...the training required...to solo a glider."

As for the 1980s through the 1990s--The vast majority of BASIC COURSE GLIDER students achieved solo. It was rare not to solo.
When did not soloing become the norm (hence the modern-day possibility to double the BASIC COURSE GLIDER solo rate)?

This is true. Back when the USAFA was training in 2-32s, the solo rate was much higher. They are now using a "club" version of the DG-1000, and the solo rate was around 40%. But by adding simulator training there were able to get that up to 89%.
-Russell Holtz