[Brian]

Gene, I know you were asking about specifically 21 studs [which are my preference as I consider them to be the best of the flatheads], however, there is a certain Canadian post war block numbered as C69A. These are the ultimate thick wall block; introduced in jailbar commercials [maybe pass cars too- who knows?] Displacing 239 cu inches, they had, in addition to the previously mentioned super thick cylinder wall thickness, the 8BA style one piece valve /guide setup, locked in shell rod bearings, no pan rail plugs [Canadian C59A blox still had them]; and were a sort of interim until the 8BA proper came out. They also only had valve seat inserts on exhaust. The original heads for them were numbered C7RA, came in both cast iron and aluminum versions. The aluminium version C7RA head was duplicated by Weiand and popularly known as cheater heads.