Hi Miles,
As I understand (fuzzily) British marking exemptions, there were three basic categories.
1. Under a certain weight (10 dwt. i think)
2. In the event that marking could damage the piece.
3. Marking would interfere with the aesthetics of the piece.
If a piece qualifed for any of these reasons, marking was left to the discretion of the maker.
Your piece could easily have qualified on two, if not all three counts. The identifying marks on it are part of the manufacturing process, part of the same die stamp that formed the butterfly and the detailing under the enamel. As to the "Sterling Silver", I've also seen it on Murrle, Bennett & Charles Horner pins that were only accompanied by makers marks. Seemingly, its an accepted practice and I'd guess legal to boot.
While I'm guessing, I've never seen anything written about, or any references, to British Export Marks. I'd guess there were none and marking was again left to the discretion of the maker.
Given that there are so many examples of exempt goods that do bear hallmarks, I have to believe that many makers chose to have them hallmarked because it gave them added cachet and increased saleability.
What I know about the subject is distilled from the little section in Tardy and I'm none too sure of its accuracy. I'd very much like to read the Silver Plate Act of 1790 & the Gold and Silver Wares Acts of 1844 & 54. Have tried unsuccessfully to track them down on the web. If anyone knows where they can be had, please let us know.
Regards, Tom
note - member "dognose" was kind enough to reply with a transcription of the pertinent sections of the Silver Plate Act of 1790. It has been given its own thread and can be found here ->
http://www.925-1000.com/forum/viewtopic.php?t=8085" onclick="window.open(this.href);return false;