While these days the release of a new model of hardware is kind of a big deal, signifying as it does important changes in components, capabilities or even a complete redesign, back in the day some companies used to be able to flood the market with different versions of their console and nobody even noticed. Take, for example, Sega's Mega Drive / Genesis.
As detailed in this wonderfully exhaustive guide over on the Sega-16 forums, Sega released new variants of the console across major world markets every time you rolled out of bed, some of them carrying only minor changes, others boasting surprisingly varied quality in terms of visuals and sound output. One had the potential to output graphics so nice it's called the "high definition" model.
There's even a model whose sound was so bad compared to other versions it's been nicknamed "the stinker".
Note that what's being discussed here aren't the kind of major changes you saw in our Genesis gallery from last year, which covered significant redeisgns and re-releases of the console. No, what we're talking about here are different models of a console that, unless you knew where to look, would appear identical, even if they contained different hardware.
Sega fans or those interested in this kind of minutiae should definitely check it out. You'll hopefully learn something; I, for example, of course knew that the PAL Mega Drive resulted in slower visuals, but had no idea it also slowed down a game's sound as well.