I don't know the answer to the early/late dates, but I have 2.
First is a 12 ga. BSS Grade II, Made in Japan barrel markings, Single Selective Trigger, I purchased new in June 1985. It had to be special ordered from Browning, so was in their inventory on that date.
The second is a 20 ga BSS Sidelock, Made in Japan barrel markings, double trigger, I purchased new in January 1986.
I have them cased in 2 Browning Leather hard cases which I purchased from a midwest supplier in 1986. They were mentioned in only one catalog, I think 1982, With Browning and FN markings, Made in Poland. As I recall, shortly after Browning introduced them, Jimmy Carter decided that Poland was the scourge of the earth and placed an embargo on any products made there.
In the Browning Catalog copyrighted Jan. 1987, Both the Boxlock and Sidelock versions are shown and it indicates that the both the Boxlock Sporter and Hunting versions have the Single Selective trigger.
Both are great guns. Hope this helps.
Gordon