Hmmmm. The 48 movement number would put it around 1958 to 1960, depending on which chart you use, so the pre-production claim would be optimistic but it does look like a cal 120 which is a great movement and (I think I'm right in saying) Zenith's first hacking movement.
I haven't seen a S58 without "S58" on the dial and the star looks clumsily placed in relation to the painted numeral - this is very un-Zenith and the bezel looks unlike the "civilian" versions I have seen.
The hands are unlike any I have seen on a S58 and the bracelet is, well, let's just say after-market.
Of course a prototype could have any parts on it to see if it works, but would it really have "S58" on the back if it was a prototype?
My instinct is that it is a franken, but that is very much my opinion. Hopefully MMMD will be along soon to give his opinion as he is our resident S58 man.
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