That's a nice early Com. Mast. you have there.

As Jack posted it has the high speed short action and hammer, the first post war new hammer. Yours has the .375" wide spur version for the target model K frames.

The standard K frame with fixed sights had a similar hammer that was straight sided with only a .265" wide spur. These hammers were nick named "fish hook" hammers.

__________________
Jim
S&WCA #819