A couple pics comparing the type I have vs what seems to be the normal plug. The normal plug does not have the extra crown after the threads and has a thicker body that would not let it fit my cylinder head.
About the only easy way known to modern science to identify an E6TE casting is to inspect the spark plug hole. Here, there is a decernable difference.
The photo on the left is a close-up of an E6TE spark plug hole. On the right is an E7TE spark plug hole.
Look closely and you can see that the E6TE spark plug hole has a deep recess from the casting surface to the plug threads, while the E7TE spark plug hole does not have the deep recess from casting surface to plug threads. This difference in the spark plug holes is the only 'practical' way I know of to identify an E6TE from the exterior.
Passenger side bottom-front corner of head (near #1 cylinder) OR driver side bottom-rear corner of head (near #8 cylinder), outside of valve cover rail...
GT = GT-40
P = GT-40P
R = E5AE
S = E6SE
T = E7TE
Could be a H.O. or a S.O. engine. They both used the same E6SE heads in 1986, and continued using them on S.O. engines after 1986.
You'll need to get the engineering ("casting") number and date code from the block to know when it was cast, or even better the VIN from the block to know what the engine is from. Of course that's just the bare block. The internals could have been changed.
For now I’m just gonna leave well enough alone and focus on paint and getting it back on the road. I’ll post back here when I swap intakes with anything I find. If I ever start upgrading the engine, these heads will be the first thing to go.