White metal either refers to a lead tin alloy or tined steel. It seems to be a cover all term. I will go back to my notes and seek more clarification.
Given the word tin comes from the american translation of de scheel, are we acutally sure that the word tin is not used in place of white metal? relying on an english translation of a book written in french, which copied a book 20years before seems a bit like chinese whispers to me. Also, some of the tables in the english translation by williams are wrong. The width of the 4-pdr carriage at the front is far to narrow to allow for the gun tube to be place on the carriage in the trunnions.