The place for the R is the callout for the Guide Key Type.
Part number without the R = Standard
Part number with the R = Another Key Shape.
Click here for the Datasheet. This information can be found on Page 3.
So basically, this is a polarizing option for the connectors. If you have a plug with this R in the part number, you must have a mating connector with the same R for the matching Keying type. Same is true if you have a plug without the R. You must have a mating connector without the R also, or they will not mate together.