Of the three non-26 arsenals producing the Type56 in 1966, at least one of them is known to have omitted the Type56 designation (五六式)-- at least, that is, on some of the carbines I've seen.
Perhaps the missing arsenal stamp is of a similar type of omission?
I think the san serif serial font was also used by all three of those tearliest, non-26 arsenals.