The strange case of the missing serial numbers

During my research I found a number of UG's with a reference number but without a serial. There are a number of possible explanations:

• Prototype: I have never seen a UG marked as prototype so probably they were never marked as such
• Sample: so used by the sales department to show to retailers
• Forgotten: my - more general - theory is that the case producers did the numbering. I would assume that clear errors like this would be rectified
• Replacement caseback: but why not add a serial
• Removed: almost impossible to do (but perhaps doable with laser welding)

Curious to hear opinions!

The examples I found:
https://marketplace.watchcharts.com...-moon-phase-chronograph-watch-ref-22538-22536 CB probably legit although the inscription on the inside is not the best I've seen. Font of the serial is off.



Strangely enough another ones exists: https://uhrerbe.com/unt/33019-universal_geneve_tricompax.html


https://www.liveauctioneers.com/item/65817824_universal-geneve-gold-tri-compax-chronograph-wit The '1' may be an indication for a replacement








https://www.bonhams.com/auction/225...compax-case-no-22536-movement-no-22081-1940s/



https://www.easyliveauction.com/cat...ey-lot-389/?utm_source=email&utm_medium=email

And another one: https://dumarko.com/collections/vin...ntage-universal-geneve-watch-rare-swiss-watch