Thanks for your comment Tony. I cannot believe the lack of knowledge of such a basic engine componant, although I must admit I haven't heard of or seen an Allen Plug. Welch plugs are readily available but if possible try to obtain the brass ones as most originals were steel. They are simple to fit, merely place them in the hole in the block with the concave surface to the outside and then expand the plug using a hammer and a drift. A touch of gasket cement around the edge of the hole wouldn't go ammis either. In the meantime read my thread on these or get that thread linked.
Bob