Welcome to the forum, Toolmatt.
This one bugged me because I could have sworn I'd seen it somewhere before, so I did some digging. Only links online I found was
a NZ tool collector site (wonder which magazine it was in?), a
classified ad from the local paper in Wellington, NZ from 1922 and an unanswered question on the Old Tools List - if no-one on there knew anything, we're in a spot of bother. So I dug amongst my own various references and the best I've come up with is in an article on picture framing from The Woodworker magazine of March 1928. Viz:
Accompanying text says
For trimming the mitres of large gilt and composition frames the trade workshops use the special mitreing machine illustrated at Fig. 2. This consists of a large metal box-like frame which carries two cutting blades about 4 ins. in width, the blades being adjusted by the usual thumb-screw motion. This box plane travels in a grooved metal bed, and the power is transmitted from the hand lever to the plane by a rack and quadrant motion. Suitable adjustable angle fences are provided to give the required mitreing angles. This machine is (and has been) in general use by the trade gilders and frame makers for many years. It is unequalled for the production of gilt frames because it takes off the mitres a shaving so fine that there is no risking of shattering the gilt and composition edge.
As to value, I came up blank recently on the same question concerning a Roger's Patent Mitre Planer and that's a much better known model, so who knows?
Cheers, Alf