RI/3 mark on M1A1 pistol grip bottom

Does anyone know the origin of this mark stamped on the bottom of the pistol grip of some M1A1 carbines? (seems like mostly second run) I have been told by some that it stood for "Royal Typewriter" (which I understand did indeed make some metal parts for carbines) and by others that it stood for "Robinson" and the "I" was for Inland. Nobody seems to know for sure, and nobody has suggested what the "3" signifies. I can't find mention of the meaning of this marking in "War Baby", but may have missed it. Any ideas?

Ray

Royal Typewriter made the metal portions of the M1A1 stock not the wood. The grip and forearm were made by Overton initially. In 1944 Overton subbed the manufacture of the grip to the Jairus Richardson Company, the R in RI. The three indicates it was a three tier subcontract, Inland to Overton to Richardson.

Hello all: Some mention of this on p. 348 War Baby. But relavant to now, look at this one on ebay:

M1 Carbine Paratrooper Folding Stock Original | eBay

Does this look like an R or a P to you on the grip bottom? I see an R.....along with the Ord. Wheel...I believe the listing mentions it is a 'P'. Would the ord wheel usually be on the side and not bottom along with the RI 3 marked grip, or does that matter, could it be RI 3 w/ ord wheel on bottom & be correct? Would\could the P be on the back edge of the stock for this one? Not on the grip side? Did they ever place P's on the grip bottom?

I have a RI/3 marked grip with the small crossed cannons on the bottom on my M1A1.
Came from the CMP that way. NO proof P on the stock. The early second run M1A1's were like that.
The first second run guns had a mix of both high and low wood stock and early and late grips. It was a little while into the second run that they moved the crossed cannon to the side. The second run guns did not have the proof P from the factory. Inland discontinued that on their stocks during the period there were not making M1A1s in late '43 early '44