A page from a Russian diplomatic cable in cipher at the time of the Alaska Purchase (Wikipedia) is reproduced in Ralph E. Weber (2013), "Masked Dispatches: Cryptograms and Cryptology in American History, 1775-1900" (Internet Archive), p.119. It was a cable of 25 March 1867 (Gregorian calendar) of the negotiations from Eduard de Stoeckl, Russian minister in the United States (Wikipedia), to Alexander Gorchakov (Wikipedia), Russian Foreign Minister. It is preserved in the American archives because it was sent through the State Department telegraph office at a cost of $9,886.50 (45,000 francs). (It is believed that this does not mean the message was encrypted by an American diplomatic code, in view of the diplomatic codes/ciphers used by the US State Department at the time (see another article).)
This was during the last stage of the negotiations and the telegram encrypted the basic conditions for the purchase of Alaska for $7,000,000. A few days later, on 30 March, the purchase treaty was singed at a slightly increased price. (The text of the treaty is found at American Memory.)
The image below is the first page, covering about one third of the whole, juxtaposed with a provisional transcription (different from Weber's), in which identification of pairs 9/g, 0/o, 8/& may need correction and x, v, v indicate some markings in the image.
The plaintext of this cable is found at NARA, RG59, T1249 "Records of the Department of State Relating to Internal Affairs of the Soviet Union, 1930-1939", Roll 0029 (fold3, p.249; the table of content on p.127) among the 45 related documents found by the American embassy in Moscow in the 1930s.
The enciphering scheme is yet to be identified.
The relatively even frequency of the symbols indicate polyalphabetic substitution is employed. (The rarity of "&", "0", and "9" may suggest these should be merged with "8", "o", and "g", making the total number of different symbols match 26.)
Frequent digrams are: 2k, dg, gf, kk, p5 (4 times), followed by 4r, 8t, dk, et, j8, rg, s7, sp, w3, x2, yd, ys, yt (3 times). The only trigrams occurring twice is 7dg (at 144th and 248th places), the interval of which (104=13*2*2*2) suggests a possibility of Vigenere of keylength 13 but it does not seem to match.
A reader who has found out the scheme is kindly asked to contact the present author.