NM Bisley FT serial # prefix

Why some have 520 prefix and some 521? Both 5.5" barrels.

520's have the internal lock. 521's do NOT have the lock. I believe that's the biggest thing. You would have to remove the grip to tell.

My 520's do not have the lock.
Tom Black

Well generally, when they use up all the numbers in the 5 digits following the prefix, they go to the next number in the prefix & start over again.
Example; 520-99999 to 521-00001

But it should be noted, Bisleys never had the lock. They just share the same serial range with the plow handle fat tops that did have the lock in the 520 prefix series.