I bought this block at the recent Snowpex Stamp Show in St. Paul, Minnesota, successfully soaking it off of the brown paper it was on. The postmark is from Chiswick S.D.O.4 dated 8 July 1976. I see that Chiswick is a district of west London, England. Does anyone know what the S.D.O. 4 means?
The number 4 is probably just a distinguishing mark, like the lower case letters in German postmarks, or the capital letters in British steel ring postmarks. I guess that this was a larger post office with several postmarking devices in use, that had to be distinguished from each other somehow.
