Definitely a replacement crown, it's way too thick for the watch. Unless someone is as much into watches as us posting here, it's hard to remember everything that's been done to it over the space of 40+ years.

The black Clebar in the group shot on OTD:

looks like it might have a replacement crown too, it's less wide and thicker than the crowns on the others.

I don't have any 12 Datos to check, but I'm presuming it has the 6.4mm crown of other contemporary Carreras - the one on the watch in this thread looks narrower than that, as well as more than 3 times too thick.

So the answer to "Crown too big?" is both yes and no - it's too thick but not wide enough. Nothing to panic about too much - 40 years of pulling out and setting time can take it out of a crown, and I'm sure some get changed because the originals are quite thin and may not give the owner much to get a purchase on. It suits the delicacy and simplicity of the watches though (though a 12 Dato is the least simple first gen Carreras get).