As others have implied, this looks very much like the stuff that Post Office Telephones, and its successors, used for plumbing joints on paper cored lead sheathed telephone cables. When I was one of their apprentices in the '60s we had a demonstration of "pot and ladle plumbing." This consisted of a pot of molten lead with a gas burner and (unsurprisingly) a ladle which was used to transfer an amount of solder into a moleskin cloth liberally soaked with tallow and held in the palm of the jointer's hand. The molten solder was then shaped around the lead sheath and the joint cover to form a water tight seal. This seemed a very hairy process to watch and there was, thankfully, no mention us lads being allowed to try it. This method was only employed when dealing with very big cables, such as those found in the cable chambers of an exchange, the usual everyday practice in the field was to use" lamp and stick" plumbing, melting small amounts from a stick of solder, as shown in the photo above, catching it in a moleskin pad and as before forming it around the cable/joint combination. This skill, like plastering, welding etc is a hand skill and not for everyone, when BT first started shedding staff in the '90s they let too many jointers with those skills go and then had to open up courses to train a new generation of plumbers. Moleskin was a thick brown cloth which was folded in many layers, the tallow came in candle form and the solder came in stick of similar proportions to the photo above and was known as Solder 8B.