There are a number of first models that are machined like yours, with the groves in the sides of the frame like yours. I track serial numbers for these guns to see if a pattern develops. It's hard to do when the sample size is so small. I will look for a thread that I started that discusses guns like yours.
Found it. Since this starting the tread below I have found a few more.
Single Shot Transition from Recoil Shield to Groved