Between 1970 and 1977, the distributor shaft incorporates an oil pump drive tooth, which engages in a slot on the end of the oil pump shaft. From October 1977, the oil pump drive tooth was moved to the end of the oil pump shaft, which engaged in a slot in the end of the distributor. Later (ie 1977 onwards) distributors can be fitted to early engines, but due to the different oil pump drive arrangements on the end of the distributor shaft, the oil pump gears would need to be changed as well. Consequently, it would be necessary to change the timing cover for a later one at the same time, as later oil pump gears can not be fitted to the early timing cover. Otherwise, all distributors from 1977 onwards are interchangeable, but may not all feature the correct ignition advance characteristics for your engine.