This Ruger manufactured late variation grip frame with trigger guard is marked on the left side "A-100" and is for the Mark I semi-auto rimfire pistol. The A-100 late variation grip frame has the magazine button flare on the left side of the frame and is in very good condition retaining the majority of its original blue finish with the balance exhibiting a brownish gray patina and containing a few specks of rust. (file photo)

Note: Ruger Mark I semi-auto pistols with the following serial numbers have been observed with this part:

12-11122 & 12-72207

16-58723

17-33228