Here are some pics of that imperfection on bottom of the frame, and the case colors look like they are in that pit. My guess is that it was a manufacturing imperfection and it was not deemed bad enough to scrap the frame (or to fix) and it was just sent on through the line.
The serial numbers look to have the same font, so nothing odd there (I think they had a different stamp for the frames).
Marlin produced an astounding 16,014 Field Grades in 1947, if they were running the plant 7 days a week that's about 44-45 Field Grades a day. I think that about sums it up, who would have had any time at that rate to do anything above and beyond just keeping up?
Based on all this, I think the barrels are re-done but the case colors and niter blue are original. The other fit and finish issues were a sign of the times. I do have to say that the gun functions just fine and lock-up and ejectors work smoothly.
An interesting look back at what was going on in 1947.
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