I don't know what brand of belt I used  for  one of my  605 J's       but the  'rubber'  has gotten hard,  and has lost the grabbiness.   Starting a core is  getting  difficult in  dry prairie hay,     These are mini rough top texture belts,  and  up until  I bought these belts,   fraying,  ripping,  was  the  deciding factor on replacement.
never had a  belt get so 'polished'  that it wouldn't form a  core.   
maybe these are synthetic, which may explain the longer  life compare to the replacement belts of 20 yrs. ago...
I'd   say   50% of the issue is   texture is  reduced,  and  50%    where the  'rubber' is  like plastic..  won't even  grab a fingernail.

It's  fine in anything with  alfalfa.   '     

so, looking at  rejuvenating,  however  temporary,  to finish out this season.    I'll start out  with new belts next June.

seen online   where   'wintergreen oil and  isopropyl alcohol'   is used to   bring  rubber products  back to life.
  Boiling rubber in water was also mentioned.. but  that was a very short term fix.   Plus,  I don't think Momma would appreciate  me  cooking  belts on her stove in her canning pot...

how about  gasoline?       Buzz them  with a  wire brush?    

If I knew if these were rubber,  or  a synthetic blend,  that may help on a fix..