These same dishes...and I do mean same(identical photos as supplied by Mingas) have been advertised on our favourite online auction site(Australian version) for about 12 months. They are claimed to be authentic 18th century and silver and priced as such. No mention of silver plate and they are listed in the sterling section.
I'm no expert in Russian silver but I've always considered them to be fake. They certainly aren't hand wrought as you can see they were spun.
Imperial Russian silver is always marked. There are, however, some very few exceptions. These object do not belong to that category. I find these objects dubious indeed and not Russian made.