Are you timing the engine with a dial-back timing light?

Looks like maybe so since you listed 55 degrees.

I have an MSD 6AL on my 32's 462" Buick engine and it was 20 degrees off when timed with a dial-back set at zero.
I think it went to the advance side.

I checked the timing with the old timing light and it was right on at 32 degrees BTDC at 3000 rpm.

Both timing lights were Sears Penske.

The strange timing bit doesn't happen to all dial-backs, just some of them.

The vacuum advance having ten degrees doesn't make too much difference since you had it unplugged when you shot the timing.
How much mechanical (centrifugal) advance do you have?

Any chance the mechanical advance weight spring went missing or broke?