There are five open wagons built now .. still have to decide on couplings although AJs are looking a good 'almost' invisible option at the moment.
A good few of us in the SSMRS used our own DIY version of the Spratt & Winkle using thin brass wire to form the hooks and loops. They were pretty well invisible, especially if blackened, and performed pretty well faultlessly. Most of us fitted a hook on one end only on stock since hooks could get entangled on tighter curves on double fitted vehicles. With the delayed uncoupling spur on the hooks, the operation was excellent. I remember operating my layout at and exhibition for over two hours without the "hand of God" having to be used.
I can't remember any member operating AJs happily. The late Robin Fielding had them fitted to his stock on his layout and I remember operating this layout at Larkrail about ten years ago and it was not easy. The couplings had been fitted in the traditional way with the hook wire anchored at the other end of the vehicle and depending on the spring of the wire for operation. I think I might prefer to use the other method where the hook wires are counterbalanced round a pivot under the body which would allow a thicker gauge of wire to be used which would help to keep the hook positioned properly. The problem on Robin's layout was that the hooks didn't maintain their correct position when uncoupled.
Les Bevis-Smith did a set of dimensions for an S scale version of the AJs which is in the Handbook.
PS. Just remembered that Barrie Norman used AJ's on his Lydham Heath layout. In fact it was Barrie who had fitted the AJ's to Robin Fielding's stock. - Jim.