Thank you all for your feedback. Getting this engine tuned is quite a learning process.
I read somewhere that the iridium was needed for nitro powered engines, but I don't know just why?
Also, I got a reply from S/S Engineering saying that their plug was a direct replacement for the SP2..
From the pictures, It looks like the "groove" is just the normal recess under the lobes for the wrench. Since HH charged me $22 for the sp2 I think I will order some of the S/S plugs at $11 and do a comparison.
My plugs measure very low resistance so I think they are not fowled, but my #3 drops out at idle very fast, so it might need to be replaced.