Plates were loaded face down into the top compartment which would hold perhaps 10 plate carriers. When the side lever was tuned, a plate would drop down into positon ready for an exposure. Then a slide on the bottom moved a roller blind inside which allowed the exposed plate to be dropped into a compartment at the back, ready for removal and a fresh plate dropped into shooting position.
The advantage of this was that plates could be added and removed at will, after one or several exposures, in a dark room.
I handled two versions of this fairly rare and very collectible camera. One is shown here, all original but with a lever on the side to operate the drop plate mechanism - all references I've seen show a brass wing nut instead of a thumb lever. I do not know which is the earlier variant but this may have been an improvement. The second example was an Austrialian import - the two Lancaster name plates had been removed and a nice black and white enamel name plate fixed to the top cover - "From J.W. Small and Co., Melbourne and Sydney" This was typical - there are many Harringtons branded wooden cameras in Australia, most of them Thornton Pickards or European but with all original naming removed excet for lens barrel engraving. Smalls survived until the 1960's-'70's.