The endings -ov and -ovo are frequently used in Russian place names. 
They all originate from possessive adjectives of a sort that are 
hardly productive anymore in modern Russian (for people who know 
Russian: adjectives ending with -ov/-ev and -in/-yn, like "otcov" 
for "father's" and "sestrin" for "sister's"). Frequently, these names 
originate from persons names (like San'kovo probably comes from 
Sanja, a short form for Aleksandr). In theory, -ov, -ovo, -ova, and -
ovy (and -in, -ino, -ina, -iny) exist (masculine, feminine, neutral, 
and plural). But for place names, normally only the masculine or 
neutral forms are used. Perhaps there are exceptions, but these must 
be very few. In my grammar book, the following examples are given: 
L'vov, Khar'kov, Kiev, Tambov, Kujbyshev, Rostov, Chernigov, 
Kishinyov (all adjectives with the (not mentioned) word "gorod", 
which means "city", and which is masculine), Borodino, Ivanovo, 
Kemerovo, Mar'ino, Kashino (all adjectives with the (not mentioned) 
word "selo", which means "village", and which is neutral).
Maybe Moskva is the exception. But I think the city is named after 
the river, and the old form is Moskova (the substantive would 
be "reka", i.e. river, which is feminine). Cf. the form Moscow, i.e. 
Moskov, which would be masculine!!
No, I haven't researched this, so I might be wrong, but with the 
things I know it makes sense I think.
Now for San'kovo: I first reported this name here at BP, but saw that 
Rolf Palmberg included it in his list of enclaves on his website 
(Enclaves of the World) as San'kova. I think the error started it's 
life there (He told me that he was aware of the enclave, but up to 
then never knew it's name).
Peter S.
--- In BoundaryPoint@y..., "granthutchison" <granthutchison@b...> 
wrote:
 > > "San'kova" is a typo, by the way.
> Hmmm.
> I'm embarrassed to see from my various files that I've been using 
> the "o" and "a" endings more or less at random, without ever 
 
noticing 
 > the discrepancy. However, GEOnet does list a number of Russian 
 
towns 
 > ending in -ovo while giving -ova as a variant, and also the other 
 
way 
 > around. (But while there are a number of San'kovos in Russia, there 
> are no San'kovas listed.)
> Is there some particular grammatical or spelling rule that makes 
 
one 
 > a better (or indeed the only valid) choice?
> 
> Grant