Any thoughts on 1962/65 Speedmaster cases with much earlier cal 321 movements ?

Hi OF,
I am looking at some cal 321 Speedmasters to maybe add to my collection, and surfing through listings to educate myself i find some odd examples that have movements earlier then the case suggest. Older movements in an earlier case are clearly later assembled i think , or just "re-using parts", but this ?

A 2998-62 case with movement number 1664xxx dating to 1959, and a 105.003 - 65 case with number 16648xxx also dating to 1959. Let`s leave out the dial and other parts for now, i am still working on that bit. I am sure this has been discussed before here for other specific watches, but i cant find a link with if there is any consensus for the general reason behind this. Possible that Omega still had some movements lying around ?

And more importantly, when buying a watch like this, is it considered legit or mostly “put-together” ? Appreciate your thoughts on this 👍

I'd say the consensus for this is that they are Frankens with likely transplanted Seamaster chronograph movements.

Thanks !
Is there a record with Seamaster cal 321 movement range numbers like for the Speedmasters, so i can check ?

Not that I’m aware of