Sega Genesis - Cover/Cart Art Variations

[eSPy]

Just noticed this while clearing out a few duplicate games. Seems like there were quite a few different print runs (in North American at least) which involved changing the labels and cover art layouts. Does anyone know of a good source for information on these variations? It's kinda interesting, like the variations of the Nintendo seal of quality (Oval and Round) on NES games.

Anyways, here are a few examples that I've found.

Ecco The Dolphin

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Taz Mania

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[Hobie-wan]

I hadn't checked when I had dupes in my hands. There are some obvious ones though like the case versus cardboard releases of Contra Hard Corps and Castlevania Bloodlines.