Les advances a strong case for skepticism of it being Imperial, to be sure. Having in the past tried unsuccessfully to find a local lab which could answer the plate vs. solid question (usual answer was "sure we can....long as it is less than 2.5 microns--classified as "heavy plate"), I had to cross-check on that 3mm claim. Les wins: an outfit called "Advanced Plastics and Material Testing" in Ithaca, NY (for instance) confirmed a laboratory XRF can determine plate "in excess of 8 microns." That would encompass most any likely plating. That kind of capability is not possible with the portable scanners (such as the Niton), as I found out all too well, but I stand corrected nonetheless. If you don't mind travelling to, or shipping it to a capable lab, it looks like they could non-destructively answer the question of composition. Density would seem to be a help re the metal content, but can you get around the effect of the enamel on a PlM, without having precise knowledge of volume and expected mass of the glass? Enamel actually ends up taking up quite a bit of volume on these things.
One other thought: is that definitely silver solder at the eagles? It looks like it to be sure, in some images (see post #22), but does not in others (post #21). Reflection of the camera flash may be inducing artifact ? (or perhaps is revealing the silver presence)--I find it hard to be sure from the available shots. Perhaps Sioni can clarify it by description or better close ups (or better yet a hi-res scan, as Les once taught me).
One other thought on the high weight. The eagles are unusually very large. If they were hypothetically solid gold (which can be verified after all, as noted above), could that move the weight of the whole piece up closer to 30 grams and still allow for a hollow body? Presumably, a lab able to do deep XRF could also ultrasound test it for hollow construction.