Measure the circumference carefully, then read a lead-loc packet or give them a ring. Chances are you have 1/2" either 6lb or 7 lb. About 21mm diameter I think.
If you get a 6lb one and your pipe's 7lb, you can "shave " it to fit.
Measure the circumference of the lead pipe by wrapping some cotton thread around it, then measure distance of thread. From this you calculate outside diameter of lead pipe (diameter is circumference x 7/22). Then you can match this to the leadloc fitting.