Hi David,
Thanks for posting those photos, I think I may be able to shed a little light. Mysterious and hitherto unnoticed small C/M inspection marks were noted in several hidden (internal) locations on 1906 Navies in the 8000 to 9200a range. These would have been assembled in late 1907/early 1908. It's possible that your 1906 Commercial was assembled at about that time. These marks have been recorded in several different internal locations and it is thought that they mark the completion of some minor change in machining procedure. The marks on your gun are the first I have noted in that particular location. I enclose photos of a couple of those hidden marks.
Norm
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