If you have birds from McMurray or any other hatchery you likely have Midget Whites.
Murray McMurray does NOT have authentic Beltsville. They falsely mislabel what they have which are Midget Whites NOT Beltsville. There are no commercial scale hatcheries with access to authentic Beltsvilles at this time. They are very rare and only exist in private hands.
There are only a few who offer them on the market right now. There are actually three lines or strains of Beltsvilles left. Brian Tibbot (former USDA) calls them Ames, Albertsen, and Ontario. Jerry Poole and others like S and S poultry have mainly the Albertsen strain and mainly stick to the Standard. Pam Hogan was the caretaker of the Ames line up until 2011 and she was very passionate about preserving the Beltsville’s original capacity as a meat production bird in addition to the Standard of Perfection. The third line is in Canada and only used as laboratory birds. I have been told by a breeder there that the line has drifted from the original bird and breeders are trying to build that line back up. Canada also has more access to the Ames line than in the U.S. Most of what you will find access to in the U.S. right now is the Albertsen line. Since Pam Hogan stopped working with the Beltsville the Ames line isn't widely available.
I love the Beltsville too. I have a pair from the Albertsen line (Jerry Poole) and 13 of the Ames line (Pam Hogan). My Ames birds are like medicine balls with leggs. Small, broad, heavier than they look but can still fly up to roost, go figure. Great gentle non-agressive keepers too. Oh, they also outlay my chickens! Can't beat em.