NPM 'U' code markings

Recently picked up an NPM 'U' coded carbine, s/n 4,369,XXX (shot it yesterday, it delivered the mail just fine! ). The marking in question is on the front of the receiver. Mine has 'CAL.  .30,M1'. Just wondering about the comma. I've read/seen it on UN-QUALITY's, but can't find it in reference to NPM. Maybe US&S did it that way on all the receivers they made? What do you all think? Thanks, Lou.

Yes. The comma has also been seen on some early QHMC carbines as well. It is possible that US&S copied the ring markings from one of the early Quality Hardware carbines.

there are a few threads on the comma

here is just one

If you look at a US&S receivers you will see they may have had two receiver variations to copy