1942 Ithaca M37

I inherited a Ithaca M37 form my Father who was an avid gun collector. The receiver and barrel have the serial # 521XX and the butt stock's serial # 581XX. By looking on the internet it appears this is a 1942 gun and the stock serial number falls in the trench gun range. The receiver has hunting scenes engraved on both sides but the left side of the receiver is stamped with P and US, the barrel is stamped with a sideways Bomb and a P, the barrel measures 20". Can any one tell me if this is a contract riot gun or was it acquired under the procurement program and maybe cut down.
Any information about this gun would be greatly appreciated, see enclosed pictures.

https://www.milsurps.com/images/impo.../M37004A-1.jpg

https://www.milsurps.com/images/impo...9/M37010-1.jpg