That's the worst commutator I've ever seen! A no-hoper, the bar ends have separated from the insulation. In days of old, when time was cheap and mica was the insulation, that would be screwed together with a big nut and chevron washers so it could be taken apart, bad bars replaced, bad mica discs replaced and the works resoldered. Today, the whole assembly is shot in place with plastic. No disassembly possible. The only thing that can be done these days is to resurface the bars and that one's well beyond any help that would give. New one time.
For those still undercutting, this is not needed with this type of assembly, actually, it's undesirable. Junk collects in the slots and shorts the bars out. When mica ruled, you had to do that to keep the mica from whacking the brushes and breaking them as the surface wore down. With today's plastics, the insulation is softer than the brushes and undercutting slots is not needed. Wears flush along with the bars. Anything more than just scraping burrs off from turning will give trouble.
Rebuilts vary from vendor to vendor. Some joints just replace the brushes, sand the commutator, maybe replace bushings if they look like they need it and shoot a coat of paint over the works, pits and all. They depend on folks dumping the vehicle soon after to avoid excessive returns. Others will replace other parts as needed and you end up with something approaching an OEM new part. NAPA stuff should be good. Price and warranty is an indicator. I like to disassemble rebuilts and relube with my choice of grease. Also lets me check out the windings. Had one where the field strapping was just rattling around in there with brown wrapping paper for insulation. Only the packing string they used for tying things together kept it from total meltdown.
Stan