Experimenting with a cheap bike. Generally, just seeing what fits, then seeing if it has any benefit at all. (Including looking odd in a good way - it's helpful if the local cops can recognize all my bikes from a distance, since most of them already know me and take interest if they see someone else on one of them.) If not, it goes back on the spares or recycle piles.

Actually, I figured that part out; it sticks out just enough that a file held sideways in the vise will engage the notches to remove it. It's already using an 8-speed chain, since that's what was available locally, so the only real reason I haven't tried a 6-speed freewheel on it yet is that I haven't gotten around to fishing a good one out of the pile.