FWIW, here are actual measurements I made of combustion chamber volume in various Commando cylinder heads (in the head only, not including piston, head gasket, etc.) back in 2009. I've highlighted in yellow the ones that might be of interest to you.
Many of these heads have been modified a number of times, including milling the head surface, installing different size valves, and opening the counterbore for larger bore sizes.
Jeff Law’s 750 head converted to 920. Milled and counterbore machined – 44.6 cc.
Steve Shivers’s 750 head converted to 920. Milled and counterbore machined – 45.8 cc.
Head from Marc Field’s Dunstall 810 engine. 850 converted to 750 bolt pattern, milled. – 42.4 cc.
850 head from Jim Schmidt’s race engine. Milled. – 42.4 cc.
Axtell 30 mm port 750 head – 51.0 cc.
Axtell 32 mm port 750 head – 48.8 cc.
Stock Mk3 850 head – 51.6 cc.
Mk3 850 head milled .060” – 43.2 cc.
Stock Mk2 850 head – 51.8 cc.
Short Stroke 750 head – 55.2 cc.
Another Short Stroke 750 head – 52 cc.
Ex-factory Flat Track Short Stroke 750 head as removed from John Hately’s bike – 57.3 cc.
Ex-factory Flat Track Short Stroke 750 head after milling – 50.7 cc.
Nominal factory spec for standard RH4 850 head is 50 cc, and for RH7 short stroke head is 53 cc.
Ken