If I understand Noel correctly, I think that we have reached the same conclusion, that the Winch, Derrick and Engineer (either as a type or to describe the users) bodies are all minor variants of a single type. Again, I think there is more difference between the first and second types, due to winch location, than between the variants even though the Body Parts List refers to them as separate vehicles.
For Bob C. (say it fast and it sounds like he's one of the Bobsey twins) I think there were quite a few vehicles that used similar derricks. Examples include field expedient M3 half tracks, DUKW, FWD YU (see Observers Fighting Vehicles Directory WW2 (1969 edition page 69) and I think Ive seen photos of others.
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