SMLE Craddle Arm, WWI on GB

[Beerhunter]

It's an aiming stand. As far as I know, mainly used for instruction- the recruit would get his sights set on a target and step away so the instructor could check his point of aim. I don't think rifles were fired from the stand. They may have been used by armourers, but it would be very limiting to what you could do to a rifle while on it. There was a later version that suited the L1A1.
Mine is missing the elevation adjustment assembly. It's also made with a fixed steel tripod, similar in appearance to the wooden legs on Badger's excellent example.

Quite correct. As you say, the tripods of the later ones are made of steel and do not have the elevation screw. The correct term is Aiming Rest.

They are too high and not stable enough to even clean a rifle on one let alone do any meaningful work. That is not to say that they were never used by armourers, its just that that is not their purpose and they wouldn't be much use anyway.