Jay - I knew you would know what I was referring to :biglaugh:
NGK 5275 is a platinum plug...it is an OEM spec.
NGK 5689 is the iridium plug.
NGK 2330 is the copper plug.
NGK uses stock #'s (above) or part #'s...for example the BCPR6ES is the 2330 plug at stock heat range...a BCPR7ES is one step cooler...a BCPR5ES is one step hotter.
BTW - the Autolite 3923 is shorter than stock by ~1/8"...it can cause arcing issues with some aftermarket plug wires as a result.