Case is not that crispy. Someone's tried to salvage a savagely overpolished case by putting some edge on it, but it is a sad has-been of a watch - the lugs have lost a lot of mass. Here's how it would have started life:

 

Ref 19518 I believe. The reason I hedge is that the one in ELV web's thread starter post is very atypically marked with a later Zenith case number in the ###A### format. I don't have any other cal 156s in the database so marked. Overall, a pretty odd piece.

 

Very strongly disagree.

IMO, it is not original finish. I've not seen a case like it among the cal 156 Zeniths. I will even go so far as to accuse the lister of photo shenanigans. NOte the rather pronounced lateral facet on the lugs in the first two pictures. Can you find it in the later pics (never mind that the background and light has changed? I emphasize that the three numbers-letter-three numbers case number format is also extremely atypical. The dial is a redial - I have no doubt about this.

Ask the seller for more pics - bet you don't get em!

NOte the lateral facets on the lugs


What happened to the facets?

 

Ah, OK. The pics I posted are from an Italian sales listing that I mistook for the original but is obviously a scam. Suggest you include a link in the future to save us from Google Image errors.

Due respect to Matt, who generally vets his pieces pretty well, but my original option stands. That is a redial, lapped to within an inch of its life. Note the case hallmark usually present at 9 o'clock is obliterated. That he is selling it for less than what one of these in good shape would bring on EBay, and that he studiously avoids the word "original" in reference to the dial should tell you he has some awareness of its shortcomings.