Terminals can be marked in a number of ways, which may be confusing when interpreting old circuit diagrams. The 'line' wires connecting telephones are taken to the terminals marked A and B, La and Lb, L1 and L2, or L and E. (L stands for Line and E for Earth). The battery connections for local battery telephones are marked C and Z, MC and MZ, or more obviously + and -. Z stands for zinc and C for carbon, since a dry battery has a carbon rod (the positive terminal) and an outer case of zinc (the negative terminal). The microphone (M) voltage is 3 volts: anything higher will soon burn out the transmitter.
Battery call phones may have terminals marked RC and RZ for the ringing voltage (normally 6 volts). Long-distance battery call telephones require a separate bell supply of 4.5 volts, across BC and BZ. Sometimes all positive poles are taken to a common connection marked C. E denotes the earth connection, which implies that the telephone incorporates a lightning arrester or is intended for party-line working in which the bell is connected between one line and earth. ER stands for extra receiver and two terminals marked EB are for connecting a series extension bell after the strap connecting the EB terminals has been removed.
Source: Old Telephones by Andrew Emmerson
Published with kind permission of Andrew Emmerson