The blocks are shaped a little differently, which is why they have different filter plates. For some reason though, the 90 degree adapter bolts up to a magnum and works perfectly (large ring gasket on block, cork on the block side on the bolt, and copper washer on the head of the bolt).
You cannot use a LA filter adapter on a Magnum block. Inner and outer gasket flanges are milled at a different height on a LA blocks. Magnums the block gasket surfaces are the same height. Magnums and R blocks take the aluminum 90 degree adapter or the 6 hole plate. LA and X blocks take the iron adapter or the 4 hole plate