OK Ray - I see that I had not found an earlier patent. Thanks for pointing that out...

1574994 (1926) DOES mention the use of thiosulfate as a sensitizer, but 1623499 (1627), the one I quited above, DOES NOT mention thiosulfate. And Sheppard goes on to make sure you know that he's not talking about thiosulfate as well.

So why the difference in the two patents then?