From those pictures it looks like the controller must have originally required adaptors to plug into each console/computer. I seem to remember the Amiga/Megadrive/(maybe Master System) had the same type controller plug, but I think the Nintendo ones were different....
Anyway, it's irrelevant, if you only want to use it with your PC. I *think* the easiest thing to do would be to disconnect the sticks/buttons from that circuit board, then either connect them up to an IPAC (preferable but more expensive), or hack them to a gamepad (Sidewinder seems to be most popular). It may even be easiest when doing this to disconnect ALL the wiring to begin with, and start over. This is all assuming you never want to use this controller as intended for those different systems...
Someone please correct me if there is a better way

Maybe this is where X-Arcade got the idea from
