Minor nit-picking?
The DDD conversion heads are from Bob Davis, not someone named Davies... Bob D has offered the teflon disk heads for Cox 1/2A engines for over 30 years... Sells excellent fuel, too.
Another conversion head is available from RJL, in California. Theirs is an O-ring sealed contra-piston type, and comes with a locking-bar for the comp screw.
As it fits the Cox glow-head threads, I tried the RJL head on a few NorVel (aluminum fin, not ceramic) .049 and .061 engines. A bit trickier starting than the teflon DDD heads, but they don't burn through. As I recall, I could turn a 5.5-3 over 18,000 RPM before I chickened out about the settings... (My own home-blend: 30E, ~41K, 22Oil, 1 or 2% DII.)