Anne, the Jacobean suite was made by both Inwald (in Czechoslovakia) and also (later) by Davidson. Inwald called the pattern "Lord" and it was also marketed by Markhbeinn (France) who called it "Milord".
Clayton Mayers were agents, wholesalers and distributors. They were experts in publicity and advertising and mounted some amazing campaigns in the UK to promote the extensive and hugely successful Jacobean range. They also acted on behalf of Inwald to register some of their designs. Details about this are in my article.
As a quick rule of thumb (before searching through catalogues etc), to determine if an item was made by Inwald or Davidson, I use the "mirror shiny base" test. If the base of the piece is highly polished / ground and stunningly mirror shiny, then it's almost certainly the Inwald version. The mirror shiny base is a characteristic of much of Inwald's high quality pressed glass. By default, if it doesn't have that characteristic mirror polish, then it's most likely Davidson.
I think I have a twin to your jug, and from memory, I think it is a Davidson one. Nice piece. :lol: