The one closest to the camera looks like its a 38 inch mower deck subframe. The farther one looks like a 44, 50 inch mower deck subframe. The manual has really good pics. I've been through them all, trying to figure out what I had on my cub. I couldn't see enough of your mule drive to figure out which one it is. If it's for a 38 inch deck, it will have a spring or possibly a nut on the left hand side of it. If it's for a 44,50 inch deck it will have an idler ratchet on the left hand side of it. I'd try using the 44, 50 inch subframe for your 42 inch deck, and try the 38 inch subframe for the 38 inch deck. The fork for the 38 inch subframe should go around the platform support shaft, and the fork for the 44,50 inch should go around the tractor lift shaft.
http://www.onlycubcadets.net/pdf/38_...p%20Manual.pdf