Possible mistake in PL and the controversy have been discussed on Russian forum:
(marks on a spoon)

The statement:
Arguments, please! Efimiy Sidorov at P-Loseva I know, I know how much you love her! She indicates the years of work in 1833-1841 .. And the assayer Dmitry Ilyich died in 1844 .. Explain to me the gap of 3 years, please ..
Аргументы, пожалуйста! Ефимий Сидоров у П-Лосевой Знаю, знаю, как Вы её любите! 1833-1841 годы работы она указывает.. А пробирёр Дмитрий Ильич шлёпнул в 1844.. Разъясните мне люфт в 3 года, плиз..
The answer:
Formally, of course, you are right ... The years do not match ...
Формально, безусловно, Вы правы... Годы не сходятся
Interestingly, they seem to fully agree assayer DT died in 1844, not 1850 as claimed by PL.
Researching Russian market I noticed that ЕФ:С is often attributed to Ефимий Сидоров but it seems they simply followed PL , so the mistake is repeated (as you have already mentioned in regard of several literature references).
On the other hand, PL was probably wrong with the years, and the mark itself is correctly attributed.
The controversy remains.
Regards