The musical instrument firm of Jul. Heinr. Zimmermann was founded in St Petersberg in 1875 but in 1886 moved its headquarters to Liepzig. Their 1899 catalog mentions branches in St. Petersberg, Moscow, Riga, and London.
Here is an excerpt from that catalog showing their flute offerings:
While three Boehm flute models (imported from France) were listed, many models of wooden conical simple system flutes were offered. They were separated into four catagories:
(a) simple system flutes with 1-6 keys,
(b) flutes with 8 keys, described as "Für Schüler" [for students],
(c) flutes with 10 keys, described as "Für Liebhaber- und Schüler-Orchester" [for amateur (flute-lover) and student ochestras],
(d) flutes with 12,13,15 keys, described as "Für Künstler und Virtuosen" [for artists and virtuosi].
I have three flutes matching or close to Zimmermann models mentioned in this catalog. These flutes are pitched at A=435. (All wind instruments provided by Zimmermann in 1899 were tuned to A=435 unless another tuning was requested.)
(1) Model 6524. Stamped Jul. Heinr. / Zimmermann / Leipzig. This is a typical late 19C German-style simple system flute with 10 keys. (Some would call it a Nach Meyer flute, but Zimmermann doesn't.) It is not a professional's flute according to Zimmermann's 1899 classification, but it plays well.
(2) Model 6535 (approx.). Stamped in Cyrillic. This is a Viennese-style flute, a full "Ziegler model" with two keys for every note outside D major and a Bb foot (15 keys). It is not quite the same as Model 6535 in the catalog which lacks the second D# lever and the low Bb. Curiously, Model 6535 has an extra key touch between finger holes 4 and 5, which may (?) be a variation on the Tulou F# key. (The usual form of the Tulou F# key may be seen on Model 6540.)
(3) Model 6544. Also stamped in Cyrillic and with St. Petersberg. This is the "Modell Zimmermann" (13 keys) that was endorsed by Ernesto Kohler. The orange bakelite rollers on the low keys indictate that flute (3) was made after 1907 (bakelite was invented in 1907). This flute matches Model 6544 except that it has an ordinary headjoint and not a Schwedler head with cusps on either side of the embouchure as illustrated in the catalog. The 6544 in turn is a variation on the original 1885 model Schwedler-Kruspe flute; it lacks Schwedler's e'''/d''' trill key and instead we find a second G# key (while the common form of a key for that trill, controlling a high hole close to the barrel joint, is retained).