hello there, im andy, quite experienced with all this.
well in all the years i have done this i have never seen that socket, what you have is the old number 3 or 6 dropwire terminating straight into some sort of master.
firstly in order to run extensions you need to attach them on top of the existing cable 2 and 5, the other numbers on the socket in this day and age do not have any function.
you really could do with a visit from openreach as there is very little you can legally do with that, tbh you should not even be running an extension from it.
what is required is a bt80 to terminate the dropwire, then a length of 4 or 6 wire to the desired position and a nte5 type socket fitted. the bt80 will be required due to the fact all new openreach nte5s use insulation displacement connectors, these are not compatible with the dropwire. unless of course you can lay your hands on an old screw type nte
these comments are my own and in no way represent any company that i may or may not be linked too.