I have never seen one with no markings on the left (date etc).
I have books at home and can look it up via other markings.
If it was later war looking, I'd think a USA GI put together.....
I disagree with John, they are well made, nice shooting guns; unless you are looking to sell, then I would double your money you paid. I would guess something like this, as a curiousity would sell for around $400, perhaps a bit mroe. HOWEVER, you get it shown or proven to be a "Familerization Pistols", used in factory training only and the value would go up a $1000 I would guess.
Nambu's have not risen in value compared to a Luger, or even a P38 over the last 25 years.
The Banzai forum could give the best answers to it.
http://forums.gunboards.com/forumdisplay.php?f=52
Ed