Will is right on the button with those IM mounts. They were made for both YS and OS .61s and made it very simple to swap out engines. My assumption here is that the designers felt that screwing the engine just to the hardwood motor mounts wasn't secure enough and they wanted metal there. I've used this mount on a Cosmos 60 and an Aurora and both worked great. I think I still have one set of parts hanging around the shop somewhere. You were quite correct to build up those aluminum mounts as the engine centerline would be a bit high without it. I need to get back to work on my new Aurora. Basically mine is framed up which means 90% done 90 % to go.

Dan
Carolina Custom Aircraft