Well, I've had time to do a bit of research in my wagon reference books.
By far the most common form of container shackle, in both pre- and post-Nationalisation photos, is in the form of a small screw coupling with chains and hooks attached; ie. the adjustable section comprises a central threaded rod / tommy-bar with both left and right-handed screws joining two D-loops. More modern versions feature a (rubber?) shock absorber in the linkage.
The shackles used for the BR CONFLATS L seem to have been slightly different, in that the centre adjustable link is threaded at either end for left and right-handed screws. I've come across these fittings in non-railway applications; you insert a tommy-bar into a slot in the central link and turn it to take up the slack.
Anyway, for my purposes I'm going to need an awful lot of small screw couplings; 3mm. scale would be ideal for my 4mm. scale CONFLATS, were such an item available. Any 3mm. scale modellers out there know of a suitable product?
It has been pointed out to me that Roxey lists an etched load shackle for use with small chain; I'll be getting hold of some to see if they resemble what I'm seeking.
Anyway, thanks to those who've responded to my query.
Regards, John Isherwood.