This unusual shaped brick appears to be designed to key in brickwork. It is a firebrick composition.Gibbons Brothers Limited, were Engineers, Millwrights and Contractors, Palace Chambers, Westminster, they are recorded as having worked on the Lowenstoft gas works (see UK National Archives).
Modern Gasworks Practice 1921, records: "In the latest Gibbons and Masters' system slotted fireclay boxes, moulded out by machinery, are erected side by side in vertical tiers so that no straight joints occur. This system has the advantage of being particularly simple, only necessitating the use of the two-standard blocks ( the online text version doesn't have the Figs. 42 and 44, one of which I guess is a corner or closer), whilst the wall-thickness separating waste gases and secondary air is reduced to 3 inches with complete immunity from short-circuiting.
These were dug up in my coburg backyard, no idea how they got there, but the vege beds had a layer of rubble under good composted soil, so I suppose they were placed for drainage.