Hi rispolilbero,Dzuo,Luis22, and Mauser99 ,
the topic of unmarked 6x30's has been raised on this Forum previously . I attach a link. The link covers black 6x30 and does not add much new info to the conclusions here - however the general consensus on unmarked 6x30's appears to be: well worn tan 6x30's are likely to be a late war whilst the more common unmarked black examples are post war.
( suggestion to Moderator: We should make this link a sticky )