Nick in Manitou, I looked at your pics of the latch mechanism and thought something was not right. I took some shots of mine which I have posted here. I am pretty sure you have an aftermarket latch which may be your issue. I see NOS or good used pass door latches on ebay fairly often. I have an NOS spare here and it looks just like my pictures.
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1947.2 GMC 1/2T SWB panel
1947.2 GMC 1/2T long bed
1948 GMC 1/2T short bed