I am thinking that you are purchasing some hype! Honda did not specify any hardware / parts differences between the engines in the R and non R NSXs. The final drive ratios were different on R cars. However, there is lots of internet opinion about the R engines having a more careful final assembly. Take that for what it is worth. Unless somebody has wrecked an R, it would be extremely unusual for an engine / transmission from an R to become available. Because of their value, I suspect that any R that has been involved in an accident is likely to be rebuilt unless it was wadded up in a ball following a high speed collision with something immoveable.
If the engine is really from an R, ask the vendor to provide the VIN of the donor chassis. The VIN will tell you whether it is an R type and you can search the VIN to try and find out whether the car is still in existence or whether it is a fabricated VIN (at least on NA cars Honda uses a check sum digit to confirm that the VIN is legitimate). If the car is still in existence, I can bet that the engine you are purchasing is not from an R. You will have to use a Japanese market VIN decoder since the JDM VIN arrangement is different than the NA VIN arrangement. I don't know whether the transmission number identifies the final drive. That is something else that you could check into.
I would bet money on this engine and transmission not being out of a legitimate R.