Showa forks have the maker's name on the inner surfaces of the fork bottoms:
A copy of the manual including Showa forks is here:
https://www.manualslib.com/manual/15...96.html#manual
P362 on covers fork refurbishment. The spec sheet says 500cm3 per leg. The detailed instructions say 500cm3 for one leg and 538cm3 for the other but it refers to both as 'left leg' so good luck working that one out! I can't imagine why the capacity would be different for each leg (unless perhaps compression damping is in one fork and rebound in the other).