Correct, those are the books. Narvas och Nyens Guldsmeder is a very thin book and less important but interesting. The town Nyen or in Finnish Nevanlinna was in the beginning a Swedish fortress Nyenskans (1300-1703) but occupied and destroyed by Peter the Great and St Petersburg built on it. A memorial statue is placed where it was on the other side on Neva river opposite the monastery Smolna. I've been there, not much to see, close to nothing.
I have a spare part of Narvas and Nyens Guldsmeder for sale. Send a PM in case of interest.
The other book "St. Petersburgs Juvelerare, Guld- och Silversmeder 1714–1870" (published 1951) is very difficult to find but here is one piece for sale
: https://cecilhagelstam.com/shop/backsba ... versmeder/
Note, it is not cheap. The text is in Swedish. It is an interesting book. Unfortunately to some parts outdated but anyway. Bäcksbacka wrote this book without having the possibility to access and research the Russian archives due to the Soviet era because it was capitalistic propaganda. In the introduction Bäcksbacka tells that. Due to the above mentioned reason and era, the books partly lacks vital information. Anyway, Postnikova is often referring to Bäcksbacka.