The pink dog has a ridge near the top of the shot glass holder. The ridge looks molded in (not just ground off).
Guernsey used the Cambridge molds until 1984, then the molds were sold to Summit who continued to produce them. Where the molds went next is unclear, possibly to the National Cambridge Collectors.
The Cambridge decanters had a glass top, but it was called a cap rather than a tumbler and was listed as 8 oz. As to how to tell Cambridge from Guernsey or Summit, frankly, it's not certain you can.
