These are not that rare, commonly referred to as Rapier missile trailers. The picture shows the metal bars on the front headboard and where they were on the tailgate. These are to hold the missile boxes in place.
Looks like it has the correct wheels on it to match the Land Rover 127 Rapier.
The handbakes commonly seize. You can get new, replacement slave cylinders off ebay. Normally a bit of a job to get the drums off though.
The suspension has some pins to hold in place are these ok? Or is it the rubber you need, I was told the big suspension rubbers are the same as on trucks, but not needed any, so I haven’t searched for them.
Search Rapier missile Trailer and you will find some info.