Well, Winn today is not your lucky day. First you live in Utar, you also live in SLC and lastly you picked me to respond.
Note: being from south idaho I am obligated to give you Utar boys some grief.
Just guessing, as I am on authority the subject, but others who are famiiar with me will check in to see what kind of bad information I'm giving and correct it.
300 Imp. (improved) Mag (magnum) could mean several things.
The only cartridge with the name 300 Mag is the 300 Winchester Magnum. Odds are that some one back in the day of Hollywood Gun Shop, wanted to do a bit of wildcatting and improved the cartridge by either creating a steeper shoulder, straightening the body or both.
Sorry that's the best I can do.
Go here for some additional info: Hollywood Gun Shop Catalog [Archive] - THR