Well I have two serious hints.

1) The eye witness has an entire box with parts he got in 1945 and kept in his barn for over 60 years. I hope there will be a serial number somewhere. It's just an hour drive so it's a matter of days to find an appointement with the gentleman and get the material.
2) I am restoring the spark plug and found several numbers on it , may be something that looks like BMW+ a bunch of numbers , but nothing positive, just a guess at this point.

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