Thank you, Foster and cowman, for helping me out. Few things have made my brain hurt like this particular research.
Although I'm finding nothing to substantiate my idea, the first bull I thought of when wondering about double muscling in Angus Was Rito NBar. He always looked weird to me.
Not only was he the sire of 707, but he was in top and bottom in the pedigree of 6807's dam.
At least one OCC bull is an M1 carrier ( myostatin mutation), OCC Prototype 847P....but that's a different story from Marshall Pride.
There is also an Angus code for DM, Double muscling, so I guess there are two different myostatin mutations in Angus?