Here is my Saginaw. It is the same story, a likely original gun with only the rear sight changed. It also has a odd heel marking in that Inland and a serial number were ground off and restamped Saginaw with a SG serial number.
There is a CCNL article about these with pics in the article that look just like mine. I can see it in hand but can't get it to show in the pics. The rear sight has a single punch mark on the rear right side.