Looks like you have an old Harken (or other) mainsheet blocks. Exactly how it is reveed depends on how many sheeves it has ... (rollers)... go to the Harken website and find an example of your blocks (5:1, 0r 6:1) and see how they are run.
I would suggest you get the seller to help you rig the boat 1 time if possible. Take notes and pictures.
I have recently purchased a home made A Cat in the Tampa area. Due to the homemade nature of this boat I have no idea how the mainsheet runs and how it is attached to the center of the trapoline. Any help would be great!