For a bike built in 1971 and a GC I would suggest 27.2mm, Very minor chance of 27.0
I would get an inside gauge and test the tube below the lug across a number of directions, all too often there is a mechanic induced distortion at the lug ears that answers the odd size condition.
In regard to the "A" suffix at the M56A, no clear answer on that one.
John Jorgensen
Palos Verdes Ca USA