Is there such a thing as a 3 digit case back code?

my first question would be............are there any UG markings on the inside caseback? Can you supply a pic of the interior caseback?

I see. Huh that's gonna be tough I think. I guess the entire case is in question. I tend to lean towards the obvious, if you have a caseback with no markings, and odd pushers that don't look UG, it's probably not UG. Have you been able to match the case style with a known reference? Even that can be near impossible just based on pics, unless it's an incredibly distinctive case style.

Also your assumptions about the case size in regards to caliber are incorrect, cases with 285 are not normally the same size as cases with 287, and even more inconsistently, cases within the same caliber will also not have the same size as one another. In short, there is virtually no consistently whatsoever with anything UG, which is part of the fun. (Once you get a bit later like the 60s there's a bit more standardization).

Regarding the case, would need to see photos, especially the front, and sides, and lugs, not just the few detail shots. We also haven't seen the dial and hands yet either.

All we can assume so far is it's a UG cal 285 mvmt.

As to the rest I'm not really understanding your language, it's not specific enough. By 285 being the same vintage as 287, what do you mean? If you had to make broad generalizations between things, the 281/283 will be in smaller cases, the 285 in medium cases, and the 287 in larger cases. But even within the same caliber you will find significant variation between case references.

So you can't just say you are going to find a "285" back, and expect it will fit all cases with a 285. There's no such thing as a 285 back. All you can say is you will find a case back that will fit a certain case reference, and it is unknown the interchangeability of that case back with any other case references, regardless of the caliber used. Same with pushers, correctors, hands, all that stuff. I can't tell you how difficult it can be to find the correct part, even when you think it should all work out, it rarely does. I'm not suggesting it never does, but just that it usually doesn't, at least in my experience.

Since it's a redial, incorrect hands, unknown case/caseback, and unusual pushers, odds are it's probably just franken, not an experimental/prototype. I don't think it makes any sense to put money into it, a different caseback certainly won't make any difference IMO. If you like it, just enjoy it for what it is and don't worry about swapping stuff out, that's my vote.

Yes