Thanks for that info Steve and you made me look at the books and online for entry lists (and the rules/grids of the day) and I recall seeing period videos of BMW 2002, Alfa 105 and Datsun 240z + Corvettes and Mustangs on the same grid - but they did run in different groups - just sometimes in these same grids.

A bit more digging found most of the groups were combined for endurance races which explains why I have seen photos and videos of the both Sedans and Sport Car (or GT depending on who and how they define a Sport Car vs GT).

All this is by the by and why I like the ERC group as they put a grid together as happened in period - Sedan's and Sports & GT cars in the same grids aka all production based cars if you wish. If they did not cars like the Datsun 240z, TR's, Marcos, Morgans etc would not have a grid to run in. Unless like you say someone was to make a grid for them - but currently why would one when ERC allows these cars to run and as pointed out by Ray already he has to combine the 2 groups already into one grid - so why take cars away from this group?