At the weekend I sorted out an oil pump for my engine. Type 1 and type 4 pumps are constructed differently. The type 4 is a better design which doesn't need a cover plate. The gears are inserted into the pump from inside making it much less prone to leaking. However new type 4 pumps are very difficult to get hold of, so the solution is to use a type 1 pump, which luckily for us expects the same stud pattern and cam position. The downside is the type 1 pump needs to protrude much further into the case (the oil gears in a type 4 pump are half outside the engine case), and so needs some clearancing.