There are essentially three types of filter fitted to Rover V8s: those from 1968 to 1976, 1976 to 1993 and 1994 onwards used on the serpentine engines. These used different threads but please don't ask me what they are but a google search will reveal the answer.
A not infrequent modification on early engines was to use an SD1 front cover that incorporated a larger oil pump and had a proper crankshaft oil seal rather than the rope seal of the earliest Rover V8s.
It is possible that your car has a later SD1 cover fitted. One way of identifying this is to see if a conventional oil seal is fitted and also compare the depth of the pump housing.
The best front cover for distributor based Rover V8s is the interim serpentine front cover fitted to Land Rovers from 1994 to around 1998. These retain the distributor but have a much improved crankshaft driven oil pump; unfortunately, the crankshaft woodruff keyway length needs machining longer and the poly-V belt driven waterpump and alternator are driven in the opposite direction so it is not a straightforward swap.
