IBM was the first to use a Birch stock. Made by Sprague & Carleton Co. starting with type 2 stocks, high wood with oval oiler cut about October 1943 in the 3.7 million range of serial numbers. Exact dates and serial numbers are unknown.
There was a perceived shortage of walnut for stocks during this time period. several other woods were tried including birch and cherry. Quite a few stocks were made that had bad spots in the wood which came out of the factory with the bad areas cut out and patches glued in. So many of those "patched up" stocks actually came out of the factory that way.
OGCA Lifer,NRA Life member, son of a 325th GIR Glider Rider
Your stock marked OI was made for Inland and if it's a light color it's most a type of walnut as Inland didn't get into birch stocks. Being that it's on an IBM means it was a replacement somewhere along the line maybe during a rebuild.