Answer:
The model 51 was manufactrued by High Standard using a Mauser action manufactrued by FN as was the Model 50. It was offered in .270 and .30-06 and had chromed bores. It was superceded by the model 51-L which was a sumilar rifle that used a HVA action and was offered in .243, .207, .30-06 and .308.
The link below shows when it was cataloged by Sears.
http://www.histandard.info/PDF/Sears%20CF%20Rifles.pdf