Kirby belts...
Short answer...
If you own the belt, own the Kirby, you may do whatever you desire! Not necessarily the best thing to do; however, it's YOUR CHOICE!!! (~~LOL~~)
Longer answer...
At times Kirby would come out w/ a "new" belt design, testing it if you will, then it would be gone when they decided on a "better" belt.
To clarify some of the other questions/thoughts on the belts currently available:
Part #: 159059 Model: 505-D80 Brush Belt (soft blk)
Part #: 301291 Model: 1CR-G10 Brush Belt (knurled)
Part #: 301289 Model: 516-G10 Polisher Belt (smooth)
Part #: 505058 Model: 519-1HD Handi Butler Belt
Then there are the belts for the Vacuette & transmission drive.
Any belts other than these, are either "old stock" or non-Genuine Kirby.
If the belts have a "stripe" on them (regardless of the color), they are definitely a 'generic' belt. Kirby NEVER has had a "stripe" on their belts. Will they fit? Yes. Are they the same quality & performance? NO!
One final note: The real issue with using the 'newer' knurled belt, is the undue additional strain (as Bud mentioned above) on the bushings or bearings, whether in the motor or brushroll. If you are NOT concerned with this, ...or you have replaced 'those' items with new ball-bearing replacements, they by all means go for it!!!!!!!!! (~~LOL~~) ...& use the much better knurled belt.
I hope this was/is of help.
Cheers & hagd all,
Bill