I respectfully disagree.
I doubt it is any sort of overhead cambox, it appears to be more of a cradle that would support overhead valve rocker gear.
I also don't think it is AJS, the classic 20's big port AJS's that had ohv had through bolts to clamp down the head and barrel to the crankcase but the supporting structure for the ohv rocker shafts were basically flat plates bolted to each side of the head. These went through to about '28
'29 & '30 had a more elaborate "box" to support the rocker shafts but the engine no longer had through bolts, they had the convention flange at the base of the barrel and then the head bolted to the top of the barrel.
The part shown has through bolts fitted and I belive is for ohv so maybe another make in the later 20's
1914 P&M
1915 Indian (project)
1930 M50 Panther
1958 M35sport Panther