You need to have a pully and cleat mounted on the frame on the outside edge of both traveller tracks (I think they're on the corner casings). The old locking types did not have these. a rope is tied to the outside edge of the traveller car, threaded trough the cleat on the outside and the two traveller cars are tried together with a piece of bungee cord.

There's a picture of what I mean in murrays catalogue

http://www.murrays.com/archive/29.pdf