The hump or curved end goes at the end where the ash drops off. If it's not that then I'm at a loss.
I wish I was home with mine to peek at it.....that does not sound right to me. I believe mine comes out and pushes up a sharp hump and then slides down the slope and falls off? Does yours push a full bed across and up the slope and then falls over the hump? Anyhoo, I was thinking the flanges of the stoker bed itself leave a low spot back at the point it penetrates the boiler wall, where the firebrick go. I felt I had fines come out because of a small crack there. If the stoker is pushed in with out the gasket in place on the outside of the boiler(going on memory) it might make a crack big enough for rice coal to get out.