I made contact with a Mr. Phillip Tom through Fernando here on the Forum. Thank you again Fernando. Phillip says the inscription appears to be the number "183". A lady at a local Japanese restaurent believes it says:
"100 and 13". And the script on the breach plug basically says the same.
Well, unfortunatly, it's not a signature

But we now know it's a number in any case. The number makes me wonder wheather the gun was built under some type of Military contract? As Japanese Matchlocks go, this one is rather plain. I'm assembling the gun this weekend and will post pics. Rick.