I tend to batch build and create five pairs from the ten supplied: five hooks only and five with loops. The only fiddly bit is getting the small 'ears' that act as the hinge, bent tight enough to the body and getting the supplied PECO track pin through four holes at the same time. A small amount of adjustment, and the result of a freely operating loop. I don't know who actually designed the fret, but it's a damn fine effort producing a cheap and reliable Bemo replacement that can be worked with or without the available magnets or simply adapted for delayed use as per CP's 09 scale Brink Valley Tramway. If you work in one of the smaller narrow gauge scales or even 3mm/TT they're well worth a try.
Greenwich couplings are available direct or from Parkside Dundas.