Which series is the Apple watch with model mj2y2j/a of?

The model number corresponds to a First Generation Apple watch released in 2015. (no actual series number was used then).

It does not correspond to any series watch of today.

It predates the watches with a series number. It's the very first model of Apple watch released.

See here: Apple Watch (1st generation) - Technical Specifications

None. It came out before Apple started using that designation.

(160629)

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(160631)