Tom, my thinking is that it is simply the fold line formed where the triangle is completed, rather than an actual bit. There is simply no reason to add a bit to this type of key. Either the triangle drives it or it wouldn’t be a triangle key, and the absence of the “bit” would make no difference, as a regular triangle key would still fit and work the “lock”. Combined with the description of the key as very old, rusted and recovered by metal detector, and that the poster isn’t a locksmith (and so doesn’t recognise these points) I still think I’m correct. Plus meter box keys are very common finds!
