Are you sure that is a toploader? I have two and either of them look like that.
I think the switch is something for emissions. I downloaded a wiring schematic for a 72 and it shows a switch for manual transmissions. One wire from the electronic spark control and the other goes to an emission vacuum solenoid. I don't have that stuff on my car, it must have been removed from a prior owner. The schematic says the switch is open in third and forth but when I tested it closed in third. I found a parts diagram at the same web site which had a lot of usefull info. the switch is number7E449. http://www.7173parts.com/images/tech/illustrations/9881.jpg
The Ford book lists a such a switch for 73 Torino and Mustang with 4-speed manual transmission.
D2OZ-7E449-A Switch, Seat Belt Warning Sensor (D2AR-7E449-BA)
I'm guessing unless you are hard-core trailered show you won't bother fixing this one.
I guess by when you get to third gear the seat belt warning goes on, I won't be trying to hook this up. I guess people get rid of stuff like this along with those key buzzer relays.