UHMW comes in many colors. Aluminum boats usually come standard with an aluminum block that your oar locks drop into, fitting very loosely. UHMW blocks have an aluminum frame welded into the boat so the UHMW block can be dropped into the frame and bolted through. Then the UHMW block is drilled slightly undersize for the oar lock shaft. The oar lock is then pressed into the UHMW block. They have two advantages: 1) they are absolutely silent when rowing. 2) They are very difficult to pull out. If I get some time this weekend I'll post a photo.