The Allison M and MH marine transmissions were exclusive manufactured for Detroit Diesel marine engines and come in the following models M15, M15L, M15R, M20, M20L, M20R for Allison M and MH20, MH20L, MH20R, MH45, MH45L, MH45R for MH. For the curious, MH 20 R stands for Allison MH marine transmission with a 2 to 1 ratio that is right hand rotation. You will find these marine transmissions in  fishing, scuba diving, coast guard, and other work and pleasure boat applications around the globe.