The later model OK Cub .049 engines with glow heads (not glow plugs) were redesigned to take Cox .049 glow heads.
Of course, that also means a number of after market heads will fit, such as Galbreath/Nelson, Merlin, Norvel, NV, AP .061, and others.
The cylinders are easily recognized by the step up in diameter, between the top three fins. Dan Sitter still has a few, and is selling them with Cox heads.
The later Cub engines with glow plug heads, retain the same small diameter from bottom to top fin. May have been a cost saving measure, and to use up existing stocks of glow plug adapters, and cylinders.
I'm currently breaking in a NOS Cub .049B, to test performance with the various heads. Since I don't own a Cub glow head, this engine sports a Cox 325 head, and it already starts easier than most of the Cubs I've run.
Almost constant rain this spring is seriously hampering my engine running, as well as flying, so it may take a while to get to the tachometer readings. No doubt I will need a bunch of head gaskets, with those high compression heads.
Bill