Originally Posted by
cyclotoine
...The nut is certainly a weird feature if you cannot thread into it. No idea what is would be for and seems an odd choice for strength.
I took the OP's statement regarding the nut differently. Not that it wasn't threaded but that he couldn't figure out a use for it. I was thinking it might be a direct mount for a fender but that doesn't make sense when the dropouts don't have eyelets. Then, that got me thinking it may not be pre-1967 as I previously suggested, as Campagnolo did not offer eyelet free dropouts back then and manufacturers rarely removed them, since fenders were desirable for early season training. The more I ponder this frame, the more perplexing it is.