"The Book of the Rudge" goes into some interesting detail about head and rocker box removal.
Seems your father is both right and wrong. !!
Eh ?
It says the cylinder head should be removed from all 3 models (of 500cc's) COMPLETE WITH OVERHEAD ROCKER BOX.
And that "the method of holding down the head is unusual. When the large hexagon nuts are removed, it will be seen that each cylinder head bolt has a small hexagon formed on it, the nuts being recessed to clear this. A spanner is provided by which these bolts can be withdrawn from the head, which can then be lifted off."
Maybe you had to be there...
Furthermore, when refitting the head, with the engine out of the frame, " it is possible to screw them in too far, so that the hexagon on the bolt does not protrude, so on the next occasion the head is to be removed it will be necessary to take the engine out of the frame."
Pictures don't show any detail of what this is all about - but it does show the head and rocker box being removed (sideways) from the engine - with the tank still on the frame, it must be said.
Strange stuff, you really had to be there ?!?