looks like a trailer on eblag at the mo. The guy on there says his is an Ifor wiliams early type before they were galvanised.
It is also the same blueish colour so may be worth a look/ring to messus Williams
search "ifor trailor" on ebay.
Pete
Thanks rtb, the lock off i was referring to is the flip down curved (to fit around the push rod from hitch head to brake actuator) plate that you put into position to stop the hitch/rod pushing backwards and activating the brakes.