(The silent b - from Latin plumbum [=lead - the metal, that is] - looks to me like a scholarly addition [like the b in 'debt'])
Incidentally, I believe that might well be. In Spanish ("plomo") and Catalan ("plom"), the 'b' was lost. But that's not the case with the Spanish adjective "pl˙mbeo" ("lead-like" or "lead-looking"). This adjective is also somewhat scholarly, not usually employed in everyday chats, but rather in formal converstaions/writing. Interesting, if slightly more off-topic than your post, Bob.
Please be aware that I'm neither a native English speaker nor a teacher.