In 1996-- 1997 Smith and Wesson deleted the square butt feature.
All model 66-5s from that time on came with Uncle Mikes grips that hid
the gripframe altogether.
This is one I bought a couple years ago.
The 66-5 has a heavier barrel profile than the previous 66-4.
They were available in 2.5", 3.185", and 4" like yours.
The assembly numbers you mention are applied using what is called
''' dot matrix'' probably to reduce cost as it is machine done and not by hand.
Here is a pair of the 3.185" model 66-5s.
One of my favorite to shoot because of the heavier barrel.
Very smooth and quick handling.
The 66-5 engineering change went right on up to 2002 when the
infernal lock came to be on the model 66-6.
Good luck
Allen frame