Thank you to FA and David for the information!
FA; your has wrote "see P-L #2599", was this a pagenumber to a book of St. Petersburg silversmith?
The most comprehensive book on Russian Imperial Gold and Silver smiths is by Postinikova-Losyeva, called "Zolotoye Y Cerebryanoye Delo XV - XX BB"
Each master listed has a specific number. In this case, 2599 the reference number in this book to that master.
David is quite right. There MUST be another hallmark at least. First, how do you know this was a Petersburg piece? Is the St. Petersburg hallmark on the brooch (crossed anchors), also there should be a gold standard hallmark on it if it is gold (most likely the number 56, probably with a kokoshnik, and either 2 cyrillic letters or a greek letter (alpha would denote Petersburg).
Please describe specifically all other hallmarks. For value, you should contact Sotheby's or Christies in New York. They will want all this same information.