The difference is the material of the bearing. 'AL' is a 2-layer aluminum bearing and the 'P' is a tri-metal bearing. Clevite made a running change to the 6-bolt bearings several years ago, but not the 7-bolts bearings. They now only offer the AL bearings for 6-bolts, but they still offer the tri-metal "P" bearing for the 7-bolts. I'm not sure of their reasoning with this, but this is how it is.
The outer two layers of a tri-metal bearing are a babbitt and a moly/graphite layer. The primary duty of the outer moly/graphite layer is to reduce friction and drag. Behind that moly and graphite layer, is the babbitt layer. Babbitt has physical properties similar to solder. It's very soft, so it allows it to conform to minor surface imperfections as well as absorb dirt and other wear particles that pass through the bearing's clearance space. Both layers are very soft and offer excellent embedability to resist damage from debris.
The Bi-metal bearings are much harder and less forgiving. They won't conform as well to minor imperfections of geometry and imperfections of mating parts. This is why they're only recommended for light duty applications and original equipment replacements.
