Sometimes I can answer the Japanese revenue questions, but I don't think this is one of those times, except to say that they're definitely Japanese, and they look more like seals from a trade or manufacturers' association, as opposed to a government-issued stamp.
I don't think there's any date on them. If 18 were a date, then in this context it would be Showa 18, which is 1943, and I think that's too early. One thing that would confirm that it's post-WW2 is that the writing goes from left to right, which was one of the modernization conventions that got put into place after the end of WW2.
Are both telescopes the same brand? If so, perhaps "18" signifies a particular manufacturer?
Hopefully someone will come along who can provide more than my idle speculation.