1950s 1956 timing marks

[Bronze]

Hi, On my 1956 the timing marks on the harmonic balancer reads "C" "5" and "10" not the usual "A" and "C". Does anyone know if this indicates that the balancer if from a different year and can i trust these marks (as much as one can trust any old damper with old rubber)?
And finally how much base advance do you guys run with? mine is perfectly happy at the 10 degree mark on this damper. no ping.