I just bought a 16 size Elgin 21J O/F BW Raymond grade 472 with a single sunk Montgomery dial marked Invar Balance inside seconds sub dial. Serial number 26.346,849. (BTW I paid 80 bucks- not running-missing roller jewel-so what). The balance looks bi-metallic and has blue hairspring. The general appearance looks like the more common grade 478 but Elgin material catalog assigns different part # to 472 Balance that to 478 balance. My balance is original to movement. There is a similar grade 472 on eBay right now that also has a dial marked Invar Balance. Is this indeed a special balance or by a coincidence are these two 472's fitted with incorrect later dials??

Yes it is different. If you look the cut in the balance is further away from the arm than usual.
Kind of interesting.

Yes, Aaron, I did notice that the cut was farther from the balance arm than what one usually sees, but didn't think much of it. Can't find any information on this type of balance in my admittedly limited Elgin literature.