The sticker under the seat is gone, no other identifying information for the model number is available on the mower. There are two numbers(one on each side under the
sticker that says 15hp 42" 44X4689 on the left and 44X4690 on the right).
I really need the diagram which shows the correct placement of the motion drive belt.
I believe that an idler pulley is missing underneath.

Hi,
Just an off-the-wall guess, but try this link:
Murray lt Parts Diagram Index for 42589X89 (http://partsandservice.com/html/Murray/lt/lt42589x89.html)
Peace,
Clarke