300 Apex /300 Pfeifer
The 300 Apex I am familiar with is based on a slightly blown out 300 H&H case or necked down 375. It is almost identical to a 300 Pfeifer I have. Apex barrels was owned by Sam May, who at one time worked for Joe Pfeifer in his shop. I can only surmise that Sam took the cartridge design when he opened his own shop in the early 50s. There is a drawing of the Pfeifer case in Dunlaps gunsmithing book. Given the above it is still prudent to make a chamber cast to make sure. It was not uncommon to rechamber and not remark the barrel. The 300 Weatherby comes to mind. The reloading dies I have for both were made by RCBS and I would contact them if and when you determine the exact chambering.