Without looking up the relevant information, I believe that the SP26 or SP26/A is a 'throw back' to years gone by and is not that much use . . . especially when there is no dedicated earth connection made to the relevant terminal of the NTE5. For example my NTE5/A, an original with the 'T in circle'
Beattie logo dating from the early 1980s, has screw terminals for the B-, the A- and a dedicated earth wire.
Does anyone else remember the original (pre-1959) block terminals? The service cable brought the B- & A-wires to dedicated line terminals, a bare copper wire running from an earthing rod was connected to a third dedicated terminal, there were shellac-coated carbon blocks between both the B- & A-wires and the earth and, finally, fuses in series with the B- & A-wires of the pair before the dedicated instrument connection terminals. How much 'protection' that gave to: (a) the local instrument (b) the exchange equipment (c) an engineer 'up a pole' or 'down a hole' is anybody's guess.