I had this conversation with BobW back when I got it in '08. The tub is a reproduction with features of both M38 and CJ3A.
Other than that, BobW concluded that it was a CJ3A with features consistent with its licensed serial number. (Since it doesn't have its original ID plate, the S/N on the registration is all I've got.) Now that I've got it torn down, we can take a closer look at things like casting numbers to see if they support that conclusion. (The engine is from a 2A, though, but it was known to have been replaced in the 80's.)
I'll be taking some better pictures of the frame soon.
-Paul