I have the longer/standard key. I took out my caliper and it looks to me like the short key would have worked fine -- the distance from the bottom of the key to the mounting plate is .1" or so, and it looks to me like that still would have enough clearance. But I don't know how consistent these headstock holes are.
If I were buying from eBay, I'd get the standard every time both for not wanting to risk getting a poor fit, and also assuming that the standard would be easier to resell..with no facts to back that up lol
I only noticed when I was checking the key today that it sits higher than the others (and I've been using it for a year and a half now)...I'm not sure anyone else would ever notice.
Good luck!
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