Mid-60s Eterna-Matic 3000 with 1504k?

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Looking to buy a classic Eterna-Matic 3000 with dauphine hands and classical early to mid 60s dial style – found one, but it’s got a calibre 1504k inside.
Would this have been produced like this, or is that an obvious frankenwatch… and if so, is that a terrible thing?
 
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Do you have any serial numbers? The 1504k was an early 1970s movement, so "early to mid sixties" anything might need a second look.
 
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Unfortunately, I don't have serials.
I was just assuming that the dauphine hands / slender silver indexes style would point more to an early 60s production date (and a 1466U cal), and that Eternas from the 70s would sport a more «modern» style, a bit like this: