I love coloured Jacobean glass Paul, and am :mrgreen: for your find here, it's super! I have parts of an amber Jacobean trinket set which Marcus found for me a while ago and he tells me that's Inwald production. No idea if the same applies to your tankard or not.
My blue Jacobean jug (also with Jacobean on it) is also considered to be Inwald due the highly polished mirror-like base which Glen tells me is an indicator of Inwald rather than Davidson.
Either Glen herself, or Chris Stewart may be able to add more, of course.