Shoemaker,
I'm not sure who made that but it's 5-6 watts max drive (carrier). Output (carrier) should be at least 250-350 watts or better. Both the 21 and 23 JS6's should work, the first number denotes the filament voltage. It wouldn't hurt to measure the exact voltage across a single filament and use the one thats closest to it. You do not want to over voltage the filaments, it will dramatically reduce the life span. If the originals were 23's then you should use that, if those are not available go to the 21's and look to add some resistance to correct the difference.
Wolf