I posted this over on collecting, but thought this would be a good spot also to look get some answers. thanks

Pr-64 70 action

The info Jim gave you is correct. All early short magnum actions were stamped 458.

Winchester grossly overestimated the demand for 458 Africans in 1956 when it came out. It was more than double the price of a regular model 70. They had a bunch of 1956 dated 458 marked actions lying around when the 338 and 264 arrived on the scene in 1958-1959. They just used the 458 stamped actions until they were depeleted. They stamped the 458 for ID purposes of the wider rails. $700 tops for that action.