Hi Adam,
On the face of it, I see no reason why the piece should not be OSP, but judging for that style of manufacture from images alone in an absolute minefield.
A lack of marks is often the case with OSP, and often just a small workman's' mark is all there is to be found struck. In this case what we see, I don't think is a workman's mark at all. The patterns on these pieces, I believe, were stamped, if the marking does not reflect the pattern on the other side then perhaps what we are seeing on the under-side is a result of the pattern stamped, rejected, flattened, and then the metal used on another item, and then re-struck, leaving just a slight trace of the original pattern showing.




If the above is not the case, and is in fact a partially stuck elaborate maker's mark, then I think we can dismiss any thought of the item being OSP.
Trev.