I have heard of them in older rotary engines and may have been a racing
plug.  I could find nothing on the NGK SD10A, however, I did find a chart
that cross referenced them to ND-Denso plugs type S29.  
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Thanks for looking around Ed.  I did some searching as well, and it does
appear that these are probably an old Mazda recommended plug, but it's
obvious that it isn't a current recommendation.  I'm pretty sure the ones in
my engine didn't come from Bruce, since they have some sort of gray paint,
or grease hardened on the electrode end.  I'll just toss them, and use the
common BUR plugs.  I've got 4 new 9's on hand, so maybe I'll start with
those.  

Cheers,
Rusty