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Good question Rick. "Dot made in England" zips were used on the earliest Irvins from 1932-3. My 1933 Irvin had "Dot made in England" stamped on the zip pullers and an unmarked slider box. These brass zips with single trunnion delta-shaped stamped sliders were used extensively until the early War years. "Dot British" marked slider boxes seem to have been first used on IAC jackets with 1938 dated labels when the production of Irvins was stepped up. The pullers are usually the unmarked nickel variety and the slider is the cast double trunnion plated variety. From 1940 the unplated fragile cast sliders started to appear and these had "Dot made in England" brass pullers.
The original Dot zip factory was located in Stapleford, Nottinghamshire and was a division of the United-Carr Fastener Corporation. I hope this helps. It is likely that the Dot British zips were made outside England otherwise why the change of name? I hope this helps.