Thanks for the links to MD's keg fittings--that was helpful to have all in one place. I printed it out.
Thanks to the advice of those on the forum, I've always only disassembled and cleaned one keg at a time to prevent interchanging posts and fittings from one keg to another. Or, that's what I think I've done...
I also don't think I've interchanged the gas in post for the fluid out post on a keg. But anything is possible, I guess.
How would I be able to tell anyway?
Given that there are three different types of keg posts, does that mean we'd need 3 different types of QD's?
I'm still puzzled how an o-ring would keep me from being able to "complete the act" of securing the QD to the post. It seems like it slides on OK (2/3rds to 3/4th's of the way) but won't easily travel the rest of the distance to allow the QD's locking collar to return to position. The QD also won't rotate on the post once it gets snug. It is damn near impossible to get on and off. The bruises on my hand took a week to go away. It shouldn't require excessive strength to get on and off. My wife would never have been able to fix this problem herself.