Pit-tanned, horsehide, M's Chromexcel: CXL M's are a variation on the classic Chromexcel leather. M's stand for "mechanical". The M's are more dense in feel and they are more grainy in texture than standard Chromexcel. M's were purposefully tanned with the intention for use in motor seals and gaskets. Most of these seals and gaskets were used in tanks and service trucks in WWI and WWII. Today motor seals are mostly made from synthetic materials.