Cool Willie, thanks. Usually data plates are identifiers of a particular part, so I wouldn't think that would indicate thw WNr. of the actual airplane. With the prefix 262, it suggests to me more that it's a part number specifically for the Me262- i.e., part 601.002.H004 for the Me262; I had a Revi C12D sight from a crash once that an expert friend identified for me as being from a Ju88 night fighter because it had a number painted on it that began with 088. Granted that's not quite the same thing, but it does seem likely the 262 on the plate is indicating the airplane type. Unless there was one with the WNr. 262 601 that is.
One does see what I expect are WNr. written in grease pencil, chalk or whatever on vehicles being assembled though, so that's why I wondered if the '121' were that; since all the birds in a line being built would presumably have the same prefix numbers, only the individual number- 120, 121, 122, etc. would be necessary to write on... maybe? LOL
I wondered about the 004 indicating the Jumo, but it's actually H004, which is a little harder to fit.