I knew I'd seen it somewhere, it is on many of the pictures of Cuthbertson land rovers. The idea of it being involved in the steering makes good sense, I just don't know why the pump handle (if thats what it is).
From these pictures, I would say it is simply a reservoir for the hydraulic steering. Since the Cuthberson conversion appears to be pretty much a series of prototypes, it was probably scavenged from whatever source they had. I don't think that is a handle, it is simply a filler neck on the reservoir.
This one appears to be similar, but with a more familiar shape of a rectangular reservoir.