A tin of rosin. Crush some up and mix with isapropyl alcochol to make liquid soldering flux. What a nightmare! Sticks to your hands the bench, anything else it touches. Soap won’t budge it. Used up a bottle of IP alcohol just cleaning the mess up.
Latest repair for another telephone exchange enthusiast. These provided metering pulses for the old analogue exchanges circa 1980’s/90’s Total of 4 blown up chips a 7 segment display and a zener diode needed replacing. Note the PCB track that acted as a fuse!