Original probably was a bladder
I have an H37 - 1979. By 1979 they had changed the H37 to a bladder - slightly better than the aluminum tank under the shower seat that was impossible to get at.The bladders do work if you mount them on a bulkhead. They don't have a lot of capacity, but mounted on a bulkhead, they will give a air pocket to allow the proper treatment of waste and keep the odors at bay.I am going to switch to a tank - to get more capacity. Just measure the space and see what will fit given the thru-hull, hoses, y-valve, etc.