Yeah, I definitely know enough to cause myself problems. I measured my old headset but the battery in my calipers was dead and I lacked any other watch batteries. If the analog markings on the calipers were correct, they looked like it was much closer to 26.4 than 27 (and the new headset I had laying around and will replace it with has 26.4 pressed into it), so I'm guessing the frame is ISO (head tube is masked off as I'm repainting the frame at the moment.

So now, I'm waiting to hear back from the people selling the fork as to whether it's ISO or JIS. Since both headsets I have access to are apparently ISO and the frame must be ISO if the headset I pulled off of it was, it sounds like I need an ISO fork, and that's that. Now, if these folks would just get back with me, I'll know.

I had just seen where people were only selling the crown race and thought I could just use that if it was a JIS fork, but since what they're selling isn't for my brand/model probably (the one I pulled was OEM on the bike and not specified and the new one is old but pristine DA - many generations ago) that's not an option. I didn't know how standardized they are. Sounds like they're not within brands.

I guess I'd seen this table on Sheldon Brown's website and thought it was possible to do more mixing and matching than it really is.