[Lucien Nunes]

I don't think the connection of a supplementary lead through the inspection hole in a plug would have been sanctioned by the manufacturers because of the lack of mechanical anchorage. If an additional single core was required alongside a 2-core flex it would have been better to route it though the cable clamp too. One extra function that the hole often served was as a locating keyway for a projection on the cover. The MK 646 has this and two projections near the cable entry to prevent rotation of the cover.

In the 1930s GEC were advertising plugs to BS372, the earlier standard for round-pin plugs displaced by BS546, with visible earth connections. Other connector types with this feature included earthed bayonet adaptors, appliance connectors and the early industrial plugs mentioned above. It was not limited to plugs, see pic below of 3-pin GEC Landor sockets with exposed earth terminal that can be checked for tightness without opening the cover. The associated plugs made to a similar high standard offer the advanced safety feature of insulated pins, like the Sanders products mentioned in a recent thread, and an earth inspection hole although this is not visible in the pic. At the other end of the scale are the Reeves 2A 'Crescent Junior' low cost plugs and sockets, c/w inspection hole in the plug.

Lucien