1603/Frankenwatch question

[Quattro Libre]

Howdy,

I came across a 1603 on the bay (I know, I know, I shouldn't even consider buying a watch there). It looks quite good but I noticed that the caseback is stamped 1600 and not 1603.

Would it be typical to replace a caseback with another model number or is that a sign of a potential Frankenwatch? It otherwise looks fabulous.

Thanks for the opinions.