How can I tell the difference between L, H, and J fixtures?

The overall look is exactly the same.  They differ in the way they each fit into the track.  The adapter of an “H” fixture has three silver colored contacts – two on one side, and one on the other.  An “L” fixture has two silver contacts shaped like a propeller.   And the “J”  fixture has 2 rounded contacts and one side is marked “P”.  The “J” fixtures will fit our “J2” two-circuit track, by pulling up the contact marked “P”, and inserting a “J-CLIP” to hold it in place.