Help ID stamps on back of large tray!

[cfoley]

I have a large silver plated tray with stamps on the back. At first I thought it was British, but the shape and orientation of the hallmarks didn't match the guides. Also, there is no letter stamp. After that review, I thought it that it might be an early American piece trying to mimick the British hallmark system. If you could let me know what each symbol means and the number 28 I would appreciate it. Please let me know your thoughts.

Thank you in advance for your research.

[2209patrick]

This mark is very similar to one used by E. G. Webster & Son, New York.