Double socket necks are bad news to try to find a decent match for.
Yes there are neck folks who can make a replica, or even just a replica double-socket tenon...but the former is gonna cost a LOT of $ (over $500 easy), and the latter will not be cheap just for the double socket part (probably like $150) and then you would want to find a decently-pitched replacement neck (let's say $100 just for kicks) and have a tech replace the tenon of the replacement neck and fit the tenon to the horn (another $80 or so).
So...$325ish absolute minimum to whip a new one up out of an existing standard neck (of appropriate tube specs) or $500+ for a bona-fide replica replacement.
What I would do in this instance (and HAVE done):
Find the appropriate replacement neck , just one with a standard tenon, and replace the double-socket receiver on the horn with a standard single socket/thumbscrew one. Probably a $120 tech job plus cost of replacement neck...so a bit better than the other alternatives.
If the neck is matched up well (see vid below) the horn will play nicely and you will have a nice horn there. The downsides being - 1) if you ever intended to resell it, it would take a MIGHTY hit in market value (as in at least 25% less than a Zeph with original neck). 2) it is unlikely that a conventional replacement neck (new or vintage) that was NOT of the same model horn would produce the same tonality for the Zeph as the original would have.
This is WHY you see neckless horns sitting for sale for a long time, or getting won on auction for what, at first blush, appears to be criminally low.
Replacement neck matching - this method has never failed for me. It becomes a bit complicated in your instance because you would need to find someone with a Zeph to provide you with the pitches of their neck: