And here is the tang that needs to be located at the front of the bogies. The one on the left is from the early Britannia models. Solid disc wheels and a protruding metal tag at the front (left) end.
On the right is the bogie used by the later Britannia, the Boadicea, the Nigel Gresley, Mallard and both Flying Scotsmen. It has a Phosphor bronze strip which connects the left hand side wheel electrically to the locos chassis. That strip and or the tang on the earlier version have to fit into the slot shown in the previous page.