I have a first year 32" Superposed w/ the 'Double-Single' trigger.

I posted about it on this site a few years ago. From that posting, other sites I have read and conversations with the Browning historian I came to some understanding.

1)Standard double triggers were available also.
2)Double Single lasted only long enough for a SST to be developed.
3)It was marketing issue. SST wasn't perfected but they wanted to get the gun on the market. Ref. #2

Mike