Yes on complicated. I'm a Williams and I've made and/or engraved many a personalized family gift. But not once ever I have never put "The Williamses" on anything. Don't care if it's correct grammar or not, it just plain looks stupid to me. As for 'possessive', using Chris's <(is that right?) example above, then if I engraved such a tray with "The Williams's Wooden Tray', that would be correct, yes? So then, why would it be incorrect to just shorten it to "The Williams's", since the tray will in fact belong to/be a possession of, the Williams's...?
I'm not good at English either