From what recall back then when they first came out the M50 had no checkering on it's stock and the M51 did. They were chambered for the 30-06 and 270 and made by High Standard in New Haven, Ct.

My late dad had a tool and die shop in the 1940's and made tooling for High Standard. I recall waiting in his car when he got M51's in 06 for himself and his brother. I still have his M51. He was proud that it had an 'FN' action.