Good evening long range
I'd only be guessing but I'd bet fewer rather than more.
I don't believe the 3" chambered 20 gauge was limited to just 30" barrels. Perhaps some of the experts crazy will weigh in.

I can account for 2.
Here's a 1947 2 barrel set with 30" "Magnum" barrels.

Hey Stan, is it OK if I show a picture?

The original owner was a young woman when her father took her to A & F in New York and bought her this set.
She and her father hunted Quail and Duck





Here's a second, 30" Full and Full, 3" factory chambers.

Another couple of pictures, Is that OK with you Stan?





When I acquired this one it didn't shoot straight.
Strip and re lay the barrels, restock and restore.
It was in badly need but what a gun now.

Interestingly enough, long barrels and 3" chambers are just 2 of the features available on a New Britain (CSMC) Model 21.