Today, I finally put on the rigid fork I'd bought to replace the old RST suspension on my Dawes MTB, with Aheadset, having assembled enough spacers to get around cutting the steerer. I tightened up the topcap bolt first, then the stem binder bolts, but when I'd finished, I noticed there was lots of play in the headset. I then saw a little ring of black plastic that had slipped out around the bottom of the headset, which broke off fairly easily when I pulled at it.
Looking at Sheldon Brown's headset page, there doesn't seem to normally be a plastic ring there, but on the Park website, it looks as if there should be a fork crown race, which seems to be missing from my current assembly--the bike's bearing retainer is at the bottom when I loosen everything up to check.
I can't figure out how this would be missing, though, unless the thin, broken plastic ring was the original fork crown race, or if the original one was lodged on the former fork (see photo). I tried to loosen what looks like possibly the fork crown race from the old suspension fork, but it shows no signs of budging.
Any advice about this would be a big help. Thanks. (First photo is new fork with plastic ring showing; second photo is old fork, with seemingly integrated, solid fork crown race, but please let me know if I'm wrong.