I know that machines are different from free weights. That's a given: the machine limits the trajectory, eliminating the need for stabilizing forces, and reducing effectiveness in building core strength and coordination, etc. Machines also provide mechanical advantage. So the pin position in the stack can mean anything - or nothing.
My question is what is the conversion factor? A machine with a 200 pound stack used in seated chest press mode requires a pressing force equal to some amount of barbell lifting force (which equals the weight of the barbell). Depending on the pulley arrangement, lever arms, and cams the weight on the stack could be anything. It does not really matter if one is simply trying to build up strength but what if you want to know when you will outgrow the machine? I could previously bench 300 pounds (Olympic barbell and flat bench) 5 times. I suspect I will soon, after returning to training, be able to do 5 reps on the machine at full stack (200 "pounds") but I doubt I will have returned to my free weight bench of 300.
Manufacturers are not forthcoming with their mechanical data. I can compute it after careful measurements, if the mechanism is visible, but I wonder if there is a convenient reference. Does anybody know of one? After you outgrow the max machine weight, the machine becomes a calisthenics machine more than a strength training machine; still useful but limited.