Assembly - Craftsman 139.53973SRT Ower's Manual

ASSEMBLY STEP 1
Assemble the Rail & Install the Trolley
The front rail has a cut out "window" at the door end
(see illustration). The hole above this window is
larger on the top of the rail than on the bottom. A
smaller hole 3-1/2" away is close to the rail edge.
Rotate the back rail so it has a similar hole close to
the opposite edge, about 4-3/4" from the far end. A
3-piece rail uses two back rails.