I've seen twin cam RKs with 21" front wheels, and all the ones I saw converted to 21" have dual disk brakes.
Just out of curiosity, I only found two 21" wheels for a 99 FLHR on J&P, both made by V-Twin Manf., one of which said specifically "no provisions made for brake, custom use only" and the other didn't mention anything about it, but didn't look like it had provisions for brakes either.
You can't always trust fitment listed on some websites either. Some may just be comparing the hub width, tire width and axle diameter with the original, and there are a lot of models and changes between years or versions of the same model, so sometimes places can get fitment listings a little wrong.
Quick Google search pulled up some shopping options that claim dual disk.
So, you have a few options as far as finding wheels. Obviously the easiest route is to just look at dual disk wheels for one you like. Or, you could try to make a single disk work (and I don't know what that might take).
You could also look at the wheels you do like that are single disk and look up the manufacturer directly to see if they made the same wheel in a dual disk version.
Here is a pic of my RK with a 21.if you add the fender extender get ready for a whiskey dent on the top from the triple tree an brake valve.drill out the fender holes with a 3/8 bit an tilt it from the back an all is good.2nd picture is with the fender extender.
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