Quite some find. Although a small part if the inscription is missing at the beginning, it can easily be interpolated from what can be seen on the intact portion. In full it reads:
CAES ^ AVG ^ BR͡IT ^ X ṂAGVL ^ F̣VSVM ^ OP I͡N ^ P͡ROV ^ T͡R͡EB͡E͡L ^ MAXIMO ^ LEG ^ AVG.
The archaeologists interpreted this as:
[Neronis] Caes(aris) Aug(usti) (plumbum) Brit(annicum) (e)x Magul(…) fusum op(eribus) in prov(incia) Trebel(lio) Maximo leg(ato) Aug(usti)
And translated it as:
(Property) of Nero Caesar Augustus, British (lead) from Magul(…), smelted at the works in the province when Trebellius Maximus was imperial legate.