No Irwin-Pedersen carbines were ever accepted by the Ordnance Corp. They had many problems. Saginaw Steering & Gear took over the IP plant and used up IP parts in stock, so that is why you find IP carbines. I'm not sure of the exact number of IP carbine receivers made, but it was several thousand. Not many, considering something like over 6 million carbines made during the war.
As an interesting historical side note, the Pedersen guy in this company is the same guy that invented the Pedersen Device for the M1903 Mark I Springfield rifle.