Hi, Helen, the 975 size would be a bit slack on a 950 rail and a bit tight on a 1000 cms rail, I think the isabella tolerance is plus or minus 10 cms ,so a 975 would stretch to a 985, the only way to find out is to try it on first, Ps we have a ventura marlin.bloody good awning regards S/C
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A ventura 975 is supposed to fit a rail that is between 963 and 988, at least that was the information I had when I was buying my awning.
------------- Bill
For a licence dated 1997 or later you must add together the plated max weight of the caravan and trailer, if the total is 3500 or less you can tow it. You may even tow a caravan with a MAM greater than the cars unladen mass the restriction was removed in 2013