From an old Champion catalog that I have, a "U" prefix means that the plug has a "booster gap" whatever that means. I don't know that they are still in production.
I believe that a standard R-1 or R-2 should use J12Y plugs, which in today's world likely means J12YC or RJ12YC. Now whether those are the same exact heat range as the old J12Y I do not know.
J9Y was listed as "one step cold" but IIRC was discontinued by the time my Champion book was printed (late 60's) can't check now as it is already packed up.