Hi Laura,
These are a fascinating set of marks that I never seen before, but like pieces of a jig-saw puzzle, only when you start putting them together they make sense.
I believe the item is Scottish, and made at Perth.
The double-headed eagle was used as a town-mark by numerous silversmiths of Perth.
The 'Pth' mark, at least a very similar one, I have only noted being used by only one silversmith, that was Alexander Forbes Dalgleish of Perth who was working from around 1890 to 1940. He was registered with the Glasgow Assay Office.
That just leaves 'Dow', and the above dates fit in very nicely with William Dow, who was working at Perth from c.1889 until 1926.
See:
http://925-1000.com/forum/viewtopic.php ... dow#p79774
William Dow does not appear to be registered with any of the assay offices and this is the first time I have seen, from memory, a mark that is very likely attributable to him.
Trev.