It may just be the picture, but the circled P proof on the bottom of the grip doesn't look quite right.
A possible rebuild or proof firing stamp?
The early circled P proof stamp has the leg of the P to the left of the circle. Also the round part of the P looks squared off.
Hope this makes sense........

WWII Inland M1 Carbine High Wood Stock - eBay (item 190394482965 end time May-10-10 05:05:08 PDT)



Charlie-painter777