Came across this today?

[HappyHippyDad]

I expect this will be an easy one for most of you, but as I haven't been in the game for long I didn't have a clue what it was until I opened it up!

Any ideas?

 

[Lucien Nunes]

MK did make a clock point that looked similar but with a flex outlet. It's not an FCU for connecting to an RFC - you wouldn't get the cables into its terminals - but it serves the same purpose. You might fuse down a larger radial to serve a small circuit or appliance that required protection (e.g. the bell transformer mentioned) just as we fuse a 6A circuit locally to 3A for an extractor fan. Fuses are BS646, 1, 2, 3 or 5A. It's DP because of its age, although by the 50's SP fusing was becoming standard because the ESR had declared the neutral as solidly earthed and the 11th edition was revised to align with it.

Stand-alone inline fuse units had a long history before the FCU, partly because of the tree system where fuses were located throughout the installation (i.e. not grouped at distribution boards) to minimise length and number of cable runs. A century ago they would have been a pair of pepperpots on a pattress, this was the next evolutionary step.