> "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