This is the same bushing that comes in ALL Volquartsen Accurizing Kits
Although it can be used with a Factory Ruger MK3 hammer it is NOT advised by Volquartsen (but we have done it 😉
I do not know how it would work with a Factory Ruger MK4 hammer so, tread at your own risk.
We have used these in MK1 which are basically MK2 hammers, some early first MK1 may vary so use caution.

MKII Hammer Bushing manufactured by screw machine equipment. The hammer pivot pinhole has been drilled and reamed for improved precision to help eliminate variances that can be found in the factory bushing. Black oxide finish and hardened.

This bushing is compatible with a MKII Hammer only. It can be used in conjunction with a MKII Hammer to eliminate the magazine disconnect in the MKIII pistol.