"It seems that the true Sendra Lowers have an S on the side of the magwell, so I'm told."
I just took a close look at my Sendra lower and there is no markings (i.e., "M" or "S" or anything) on the outside or inside of the magwell. The inside of the magwell is smooth (no machining marks). The receiver is marked :SENDRA CORP
BARRINGTON IL
But as stated it is a Rock Island Armory Rifle bought new by me (I did not assemble it) in Sept 1983. So if true Sendra lowers have an S (or M) on the side of the magwell, mine isn't a SENDRA? If so, someone went to A LOT of trouble marking it SENDRA.
BTW: If my memory serves me correctly, if you reverse the syllables of the word/name SENDRA you get the name of the owner of Nessard (DRASEN). Pretty sure I remember that right. Model 1 Sales and J&T Distributing are decedents of the Nessard company selling AR stuff back in the 80s..