OK, I got an answer. I first asked why the part numbers were different. This was the emailed response:
"Sorry for the confusion, the R in R-SSR-650FM stands for "Replacement". its basically a brand new PSU which is made specifically for RMA's. They are both SSR-650FM. Hope that address's your concern. If you have any other questions, please let us know anytime."
I was still a little skeptical because the person said the PSUs are "basically" the same. So I directly asked if the R in the part number meant is was refurbished, and if not, if they could confirm that is was exactly the same PSU made with exactly the same internal components. The response was:
"Sorry for the confusion, let me see if i can explain this a bit better. These are brand new units that are not boxed with retail packaging, nor does it come with any cables or accessories. This is what i meant by saying they are specifically made for RMA replacements. they are all brand new units which are all made and assembled in the same factory as retail units with exactly the same internal components.
There is literally no difference between this power supply as the ones you would find in retail other than it is missing the retail packaging and peripheral cables/accessories.
Hope that clarifies it for you. let me know if you have any other concerns. Have a nice day!"
And there you have it, it's the same PSU.