That's very interesting!! On the rear parcel shelf they've cut speaker holes in mine and I know they were cut by the previous owner mid 80's as he's a mate and I know this cars history back to late 70's, so we can assume that the parcel tray was uncut prior to that. Looking at the above photo, appears the cage mounting points on the parcel shelf is in very close proximity to where you would normally cut speaker holes, so you may be right, there might have been 'signs' in that area prior to speakers. The inside of my boot has never been painted so its original and I've crawled under there looking for signs but nothing unusual stands out other than the horrible mess of the speaker holes by the nibbler machines.
So to try and research Group E participation between say 1972 and 1978 is that difficult?
What a pity its had speaker holes cut, is it still roadworthy?...the reason why i sold past xu1's