The Platiunums IVs I've referred to are actually Bosch Platinum +4. I've used the +2 and +4 in both of my caged vehicles. I believe the RIII is stock with NGK DPR8EA-9 ??? < Verified
I've not found a cross reference for NGK iridiums DPR8EIX-9 to a Bosch plug. http://www.gadgetjq.com/spark_plug_cross_reference_motorcycle.htm
The NGK to Denso iridium is IX24B http://www.denso.com.au/sparkplug/xref
The Denso to Bosch x-ref search was vague. The IX24 (no B) search was referrenced to Bosch X4CS, XR2CS, and XR4CS. I was unable to determine whether any of these were platinum or iridium or of the combination Fusion variety.
I did find one source for the NGK iridiums @ 6.95$
I found this cross referencing at another site, from which I'll guess the XR3CS would be the mid range Bosch plug:
"Autolite 4164, 4163, 4162
NGK DPR7EA-9, DPR8EA-9, DPR9EA-9
NGK Platinum DPR7EV-9, DPR8EV-9, DPR9EV-9
Denso X22EPR-U9, X24EPR-U9, X27EPR-U9
Denso Platinum X22EPZU9, X24EPZU9, X27EPZU9
Denso Iridium IX22B, IX24B, IX27B
Champion RA6HC, RA7HC, RA8HC
Splitfire SF416B, SF416C, SF416D
Splitfire Platinum TP416D
Bosch XR4CS, XR3CS, XR2CS"
I'll defer to any experts on exactly what the heat range numbers mean.
Note that the above x-ref, from another site, does not include the NGK Iridium plugs.