I dont know and NEVER reported about the A on R5300 plugs.
I know for the A difference of the code only on R7282 series which means that they are antidetonation type (using with detonation counter) and without aluminum washer.
As StephenRC45 said and i didnt knew it,HRC has tricky washer to use them without det counter.
For the racing spark plugs that Andy talked to and are the same dimensions and same caps with the OEM i never did any search for them as the R7282 plugs excist. Andy is any possibility to talk about this NGK code BR10ECMVX ??

regards
Stefanos