It bolts up, it just doesn't work properly. I do not think we are at the point yet where the wrong parts are out in the rebuild stream as such a normal thing. You ask for a 6v starter you should get one. Is this a new thing where we need to be concerned that our new replacement isn't the same as the old?
The starter gears are not interchangeable. Unless you are a machinist, or there is a how-to article somewhere, only a few people would know how to go about this. The point is, the OP is probably not one of them and to confuse the issue just confuses people who rely on NAPA or anywhere you can still get a 6v starter.
To answer your question.. You would know because the starter wouldnt engage properly. If he has started his truck EVER with the starter he has, then its either a real 6v starter, or like you say, a one in a million chance that someone figured out how to change those totally different (shaft size, etc) gears.
