That's a keeper!
That's a keeper!
"There's a race of men that don't fit in,
A race that can't stay still;
So they break the hearts of kith and kin,
And they roam the world at will." - Robert Service
My thoughts on the matching "F" markings were to remind the owner/armorer to keep that particular slide and frame together. Certainly could be something else but that would be my guess.