Your inkwell is not Scottish. Scottish silver uses other marks than just the thistle, and those marks are not present on your piece. I think that the silver on your inkwell was made by Matthews & Prior. They use only a thistle as their hallmark. Matthews & Prior was in business from 1898 to 1904, and they specialized in sterling holloware (which your piece is). The initials "CUS" is the engraved initials of a previous owner. The other number/letter combinations are most likely pattern or part identifiers.