Aftermarket.
And having said that, I assume what you're calling a thumb rest protrudes from the grip--and the thumb rests on it.
The alternative---an indentation in the grip (left side), more or less where the thumb would be, is not a thumb rest. My collection (and knowledge) pre-dates such indentations, so I cannot speak with certain authority; but I suppose they are to provide clearance for ejecting empty cartridges---and/or clearance for a "speed loader". Grips with such indentations are factory----but came along quite some time after 1953 (I'm pretty sure.).
Needless to say, a photo would work wonders to clarify what you're calling a thumb rest.
Ralph Tremaine