They are 100% identical to the OEM Cox Tee Dee High-Compression glow heads except for the knurl (circle of stars) at the top.
We did not bother with the knurl as it would have cost another 50 cents and serves no purpose other than cosmetics to quickly differentiate it from the regular glow head. And, yes, the combustion chamber is trumpet shaped and identical to the ones Cox used to make.
If you want the OEM packages ones, we still have a few in stock as well. See here:http://coxengines.ca/cox-.049-tee-dee-glow-head-oem.html