It is believed that this is one of a series of fired clay air vents that surrounded a Walworth grain kiln. It possibly came from such a kiln at a mill in the Perthshire area.
See this link for the patent details for the Walford Grain drying kiln.
153,130. Walworth, A. E., (trading as Walworth & Co., J.). Aug. 20, 1919. Bleaching grain and seeds.-In kilns for drying and bleaching cereals &c. of the type described in Specifications 16965/94 and 28456/07, [both in Class 34, Drying], wherein the material falls down and is dried in the annular space between an inner cylinder of wire net &c. and an outer cylinder of earthwork tile-work with openings directed upwards from the inside, and is deposited on an annular ledge 10 from which it is removed to an inverted discharge cone by rotary scrapers 9, the gearing for driving the scrapers is located outside the cone. For this purpose an extension cone or cylinder may be fixed to a driving wheel 4 on the scraper shaft may be formed in two parts 15, 16 between which the wheel is fixed, the rim of the wheel projecting outside this extension, and being driven by a worm shaft 21 or a bevel wheel. The grain &c. passes through the extension cone between the spokes of the wheel to a conveyer 25.
This clay vented tile measures 23 1/4″ high, 12 1/” wide and is 1 1/2″ thick. There are 2 sizes of openings – 2 3/4″ wide and 4 3/4″ wide respectively. It has at some point been broken but ‘cemented’ back together.
The following pictures show the front and back of the tile while the 3rd shows the ‘feet’ and groove which would locate and secure with the tile below or above. Hopefully, it also gives an idea of the angle and structure of the vent