Kev,
we went to metric in racing in the early seventies, about 1973 I think, we changed to mostly mobile racing in the early eighties so there is not a big load of material on 10/15/20 metre handicaps in total. The inters in the late 70's early 80's had loads of these marks reserved for the likes of Pure Steel, Paleface ( I think he is entitled to a one word name) Hondo Grattan etc before we moved to mostly mobiles.
I think the really big handicaps that people saw years before that was probably brought about by factors such as distance...transport these days is quicker and safer ( I know my Dad's old mentor Perc Warby used to tell us stories about jogging his horses from Newcastle to Maitland and Cessnock for big events in the 20's and 30's.) roads are better etc and so the better horse were racing against locals and therefore needed to be heavily handicapped to ensure the races was not a farce.
A very good trotter Albey Logan, trained at Maitland by Noellene Kirkwood and driven by her husband Grahame who ran second in the 1979 Trotters Interdominion Grand Final at Addington used to race and win off marks of up to 100 metres locally in the Hunter Valley because he was just head and shoulders above any other Trotters here, the Kirkwoods took him away to race from time to time ( I think he won a Dullard cup at one stage) but like most people they had jobs to attend to so a life on the road tangling with the best week in week out was out of the question.
Cheers,
Dan