Hmmm - that's almost like someone added a short spur to an original 'speed hammer'. I assume the checkering that you've referred to is on the very top of the hammer (next to the rear sight).
Ruger has come up with some odd, short run, hammers like the DAO ring hammer (like a Colt Commander hammer) on the SP-101 'State Dept. ' model.
A call to Ruger Records might shed some light.
A better photo would help.
Terry T