I have been puzzling about this watch as well, as it also has a Cal 12 movement, which is correct for the Mark 6, but questionable / unusual for the 203xxx serial number.

To answer your question -- Yes, Heuer often replaced cases when these watches came in for service, and had a regular process of receiving a worn case; refinishing it; then using it on a watch that came in for service later. Of course, this wreaks havoc on any attempt to use case serial numbers to authenticate watches, or to find watches that are "as they were" when they left the factory. But half a century later, we are left with the reality that the Heuer service department followed this practice and so "it is what it is".

Jeff

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: Hello there-im hoping you can help me out ?

: I have been offered a Heuer Sifert that has a serial number

: 203974

: When I’ve checked the website on the dash for the serial numbers
: that indicates a MARK 4

: BUT

: The dial and handset correspond to a MARK 6 ?

: SO-did Heuer ever do this i.e. place a new watch in an old case ?