The same teapot with the same mark was sold on eBay and was listed as silverplate with the manufacturer being Meneses Orfebres from Spain.
(admin edit - see Posting Requirements )While eBay is certainly not an irrefutable source, it does support the assertion that this teapot is silverplate.
Are you able to share what type of test the refinery used to confirm it is Sterling? (x-ray, acid, etc.)?