Sweet looking trailer! And welcome to a great forum.
I would think any ID plate would be on the trailer frame on side of the tongue. The idiot dealer where I bought my Scamp
put 1995 on the paperwork instead of 1975. They also got the ID number wrong. I went looking for the ID tag and found it riveted on the side of the frame, on the tongue near the propane
bottle. I think this is standard and probably required by the authorities who don't want to go searching everywhere on a trailer in order to check the ID tag.
I took my Scamp
to the DMV to have the title
corrected and the DMV inspector went right to the trailer tongue to look for the tag for inspection.
Another thing might be that the ID number is stamped in the metal of the frame, although I think this is unlikely. But if it is, it may have been painted over at some time.