older masters dont have those bottom sections only the modern NTE5 ones do
if they don't then you are only supposed to use plug in extentions but those look **** so a lot of people just connect to the back anyway and the worst i've heared of BT doing in this case is pulling out the extention wiring
this is a NTE5
this is a normal extention
this is a non NTE5 master
the latter two will look identical from the outside
in this next picutre we see what is inside a master
in the case of a non nte5 master there will only be terminals labeled with numbers the A and B refer to the seperate terminals that are on the rear section of the NTE5
the surge suppressor is of no interest in the normal functioning of the line
the capacitor provides the ringing signal on pin 3 which is linked to all extentions
if it is impractical to get a third wire to an extention (perhaps due to damaged cable) then a second master can be used to get a ringing signal for the extention but this is considered bad practice unless unaviodable (see below for why)
the resistor is for test perposes and this is why putting in multiple masters can confuse the line testing equipment. if there are multiple masters (though you could clip it out of the second master socket)