The USSR and its allies had need of topographic mapping for economic and civil governmental purposes, but the military maps, having the location-based sheet numbering system and showing geographic coordinates – designed to be used for military-precision targeting – were not suited to be made available to non-military authorities.
This problem was resolved by the introduction of the Coordinate System 1963, SK-63. For this purpose, USSR territory was arbitrarily divided into 20 regions, each with its own code letter, as under:
A Georgia
B Almaty Oblast
C Baltic States and North-West
D Udmertiia and Perm Oblasts
E Altai
F Irkutsk Oblast
G Khabarovsk Krai, Sakhalin Oblast
I Novosibirsk, Omsk & Tomsk Oblasts
J Kamchatka
K Kazakhstan
L Tiva & Krasnoyar Krai
M Altaisk Krai, Kemerovsk Oblast
P Central Black-Earth Region of Russia
Q North West
R Central and Lower Volga (Kalmik, Astrakhan, Volgograd, Saratov, Penza & Ulyanovsk Oblasts)
S Yakutia & Magadan Oblasts
T Krasnodar Krai
U Kirghizia, Tadzhikistan, Turkmenia, Uzbekistan
V Bashkiria, Tatarstan, Samara, Orenburg, Chelyabinsk, Kurgan Oblasts
W Sverdlovsk & Tiumen Oblasts
X Ukraine & Moldavia
Y Kazakhstan
Examples of SK-63 sheets of parts of Latvia and Russia can be seen here