Thank you all...very interesting. I found in my research a suggestion that the number 12 was also used to indicate silverplate, here,
http://www.beerstein.net/articles/bsj-2c.htm, as follows:
"Do not assume that 12 necessarily means Loth. Silver-plating manufacturers in Germany have been known to use a simple 12 stamp into the metal with no reference to Loth at all. Several [beer stein] lids have been seen carrying that numerical mark over the years. Plating was invented in 1840 and prolific in Europe by 1860. Since there was little if any control by the silver regulators, markings on silver-plated items can create confusion."I do not see any other indication, however, that these pieces are silverplate. They polish uniformly with no wear spots. A nitric acid test on a scratched location is nice and red. Are you satisfied that this is 750 silver? Kent