I just finished up working on a Ruger Mark II Target, and this was inside the grip frame:
The diversionary gizmo mentioned above for the uncooperative hammer strut.
My gut tells me that this
"stamped aluminum part" is used in some other application, but is now being re-purposed with a milled slot, now in it, to divert the hammer strut away from being captured behind the pin for the longer leg of the sear spring.
The reason for my suspicion lies with the way this part's shape is stamped out. The profile could just be a rectangular shape for all it does, instead of what it looks like as is, so I made a variation on my milling machine, the same thickness , width of slot, and depth of slot, except rectangularly shaped,and the dang thing worked just fine, except I used a round nose endmill to cut the slot to depth.
As was posted above, the above part is indeed an answer to a non-existent problem.