What are these devices mounted on old fashioned domestic electric trunking?
What are these things (name)? 


They are sometimes called "fuse-pots".


  What purpose do they serve?


They each contain two contacts which are bridged by a thin piece of fuse wire which connects the electrical supply to wiring to a specific individual circuit. They prevent your house burning down in the all-too-likely event that there is a malfunction in some equally ancient and uncared for wiring, switch, lampholder, wiring or appliance. 



That is an antique fuseboard

See Architectural Salvage, Brighton, England.




Yours seem to be a later Bakelite version rather than ceramic.

The individual holders may have been called fuse-pots.

Note that some old fuse-boards had fuses on both live and neutral



They look rather similar to earlier 19th century fuse holders of the sort used in the 1890s.


Early Edison Fuse block in article Early Electrical Lighting in Homes
By Steve O'Bannon

"Very old Electrical Fuses", Alamy
"Germany, very old fusebox", Alamy


However these earlier ones are all screw-in fuses following an Edison design I think.




  Any idea how to open the cover if it refuses to? WD-40 threw in the towel


Please don't spray flammable fluids onto live 240V electrical installations.(my can of WD40 says "caution flammable" on the front)

I'd be surprised if there was anyone still alive with experience of dealing with these things. At least, not in my part of the world.

If I were in your shoes, my immediate priorities would be


Find and replace the protective covers.
Warn any adult occupants of this building not to touch any part of this panel.
Prevent children getting access to this area of the building.
Pay a qualified electrician to supply and install a modern consumer-unit (main panel) where the electrical supply enters the building. Have that electrician fit miniature circuit breakers (MCBs) and residual current devices (RCDs or GFCIs) or combined protective devices (RCBOs etc).
Take these antique fuse panels out of service and replace them with a properly protected junction box or similar.
Consider getting the wiring in the building removed and replaced. At least get it tested.


Having isolated the electrical supply to that panel, I would fabricate a removal tool by screwing four long screws part way into a wooden block and cutting off the heads to make short studs that fit those four holes. A small amount of penetrating oil might help but I suspect the chances of it making a difference are low. I'd have a fire extinguisher handy. Double check that power is off before going anywhere near that panel.