Hello Bob,
My Maxim MG08 sled mount is approximately 13" wide. The front legs fold up and over the mount to give a flat surface to drag the mount. Looks like it might fit your pack saddle well if the dimensions were the same. Nice piece!
Might want to give Hayes Otoupalik a heads-up on this pack saddle, as he is the man on WW1 military horse tack and equipment:
http://hayesotoupalik.com/
David