The crystal calibrator was an accessory for either model.
I think the only reason the R-100A exists is to match cosmetically, the Knight T-150 transmitter.
The might have been some circuitry changes (additional front panel knob on the R-100A) although they both used the same set of tubes. You would have to side-by-side compare both schematics and parts lists.
R-100 has 11 knobs on front panel:R-100A has 12 knobs on front panel: