I got this prototype of Super Mario Kart a couple of months ago from c4e's huge proto sale. It turned out to be a quite early version with many graphical and musical differences, for example a totally different Vanilla Lake soundtrack that Niels discovered the day before he would ship it out to me =). I've also noticed many gameplay differences which sometimes make it harder than the released version. Anyway, I thought I'd share some pics for those of you who might be interested.
Close to the Japanese title screen. From what I can see, Mario's mouth looks slightly different.
Mario and Luigi have additional parts on the back of their karts and Bowser looks a little different.
Cup Select screen is missing the orange border and the "cc" class chosen.
Lakitu makes a different noise when driving in the opposite direction and the yellow flag is missing the black cross.
Beta Cheep Cheep?
Sometimes it says "OUT" for some players in the bottom.
When getting in 2nd place, a Super Koopa makes a loop and starts spinning out of the screen when it gets hit by the champagne cork. Also, there are no cups given as reward.
There are lots of other differences I've seen as well and it's seriously buggy in some places. I haven't had time to fully play it through yet, but hopefully I'll be able to do it soon.