Mega Drive region lockout, which model?

[Elusive]

It's common knowledge that any game produced for the Mega Drive / Genesis after July of 1992 will most likely be region-locked, due to the introduction of the TMSS chip.

What I'm interested in, though, is the original model of the console, the one that came with Altered Beast as a pack-in. It had no region lock of any kind - is there any way to distinguish this model from the zillions of others without actually trying a game? i.e. are there any distinguishing marks or features on the console itself?

[anagrama]

The first-run models had no license check (the screen that says "produced by or under license from Sega Enterprises"), which meant only that the early unlicensed Ballistic/EA games would play on that and not later ones.
There was a long thread a few months back where we pinpointed how to identify the early ones (iirc, the serial number starts with "39")

The region lockout is entirely seperate, and region-locked Mega Drive carts will not play on any type of unmodified Genesis.

[anagrama]

D'oh! Didn't notice you're in the UK - ignore what I said about the serial number, that's just for the Genesis.

I believe you can spot a first-run PAL Mega Drive if the model number is MK-1600-xx rather than MK-1601-xx.

Even so, you still can't play region-locked imports unless you mod it or use a convertor.