To add to what Jack said.
I can't speak for other engines, but all the A series engines from the 948 to the 1098 to the 1275 have to have some sort of crankcase breathing beyond the valve cover, and when the lifter covers were elminated in the 1275 to make a stronger block for the bigger displacment, all you have to get crankcase gases out of the block is the very small 1275 pushord holes, and that is not nearly enough, so the factory had to find a new place to breath the block, the vent can on the timing cover was their answer. If you use a early no vent can timing cover on a 1275 A series engine, chances are you will spew oil out the back, not a good idea, trust me this something I've put to the test over the years building these engines. You have to breath the A series somewhere on the block. If you want the look of the earlier timing cover, and your block has the bolt on mech. fuel pump blanking plate (not all 1275 engines do) you can do this, use the 1098 rubber seal timing cover (no can) and then get one of those aluminum breather blocks Mini Mania sells that goes where the mech. fuel pump blanking plate goes, that piece is baffled and allows the use of a barbed or AN fitting for hose to breath the crankcase directly from the block, I've used them on the race engines. Bottom line, you got to let these engines breath or they will force oil past the rear scroll, and possibly even blow out gaskets.