Unusual coupling

I just saw the topic heading and was looking forward to some completely different content :

Looks like the ball does all the work and the ring is a cosmetic thing to tidy it up - note the circular wood packing and that the ring is nailed down. I am guessing that there was a circular flexible skirt that went between truck and trailer to seal the gap.

mate I reckon ex military type of thing not off an artillary gun maybe made from a big 4x4 hub looks like the ball is welded on but I would have thought not I reckon I have seen one of these before just cant think were dont fail me now brain

Paul

maybe Rodders can help might off some type of train rolling stock ??????????

Think it was fairly common on the semibuses.
Fowler Road Machinery did sell a lot of bits for them. Have got a copy of quote for some trailer bits for a semibus.

I have to ask-what what keeps it on the ball?

I believe a big bolt down the centre...