Nowhere do I see a date regarding what YEAR this took place, but it seems it
must have been 1984.
Because May 20th fell on a Sunday that year, and the only other year it did so
within the lifetime of +ABL was in 1990, but by then, +Williamson was a bishop,
and he signs the letter on the St. Thomas Aquinas Seminary web page as "Fr.
Richard Williamson." Also, the letter says +ABL was 78 years old. He was born
in November of 1905, so he was 78 in May of 1984. Two different methods give
the same year, so it must be true.
I made a post containing curiously similar sentiments on another thread a while
ago. It's the last post on page 4 of
this thread.