[Kevan]

I think you read my post correctly, the 4 eras I mentioned would be 4 different proxies each split into 4 different events as you've been doing already.  Four is obviously one too many for a 'triple crown' so one would go which would probably be the pre 50's Historic as they'd mostly have to be scratch built which would put most people off.
A Classic 50's & 60's as you've suggested for the first of three in the 'triple crown' with the usual 4 events would be full of many varied and interesting cars.
I'm sure a few of you are enjoying the Silverstone Festival streamed live this last couple of days, yesterday was 10 hours of qualy action with an unbelievable number of very famous race cars from the past which has been the best thing I've watched on the telly for years (just got a 'smart' tv so watching on that).
What I didn't realise before was just how enjoyable watching such a varied mix of race cars is and this proxy idea of yours Graham is along the exact same lines.