Bee's 48G733 with 'E' suffix:
Another 48G733 this time with 'ET' suffix, and adjacent to the serial number instead of the engine type, the indented letters are said to be the engine builder's initials:
Nigel Brown's, very similar type and age but with an 'ER' suffix, same engine builders initials, but definitely flat core plugs. Something else of note is the very similar surface coating on this and the one above (more evident on the originals I cropped down to these), unlike the gold paint on mine:
However looking at the one above there does seem to be space to put flat type over the top of cup type! With mine (top picture) the outer edges of the cup-type are virtually flush with the face of the block, so no room there.