Omega Marine Chronometer 1516 Megaquartz

Most of these Megaquartz watches have the serial number on the plate, but this one in only says quartz. Any info on why this is?

Probably fell off or was built from spare parts and they reused a plate from the "Mariner" series. I remember a guy selling one with a plate he'd had engraved with the serial number and it didn't look right.

Apart from prototypes I don’t believe it can be a Marine Chronometer without a serial number.
Are there other photos including the crown / side and if possible the movement.
My first thought was also some sort of cut and shut. Interesting if it is a 1516 movement as parts are getting thin on the ground.
Maybe it is just the plate which fell off and got replaced as suggested above. Hard to tell from one photo which is an odd format.

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cambiaron el vidrio

No an OMC glass would not fit on a "Mariner II" it's much smaller and the bracelet isn't the same. For the most likely explanation see posts 2 and 3 above.