Begiining in 1839 J. Prime of Birmingham England used the marks in my first picture below. He was a close plater.
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Around 1844 they began producing electroplated wares and the name of the firm changed to Thomas Prime & Son.
http://www.925-1000.com/silverglossary2.html" onclick="window.open(this.href);return false;
This is only my opinion, but I believe your mark was sort of a transitional mark when they changed over to electroplating.
Pat.