Wikipedia article
http://en.wikipedia.org/wiki/57_mm_AZP_S-60 states
96mm @ 1000m
106mm @ 500m
(no angles so I'll assume its at 0)
A little playing with a test gun in MBT_APCALC seems to indicate that a base pen of 12 gives reasonable approximation of those figures, i.e. 11 at 500, and 9 at 1000 for the reliable penetration line. (Currently it's getting 15 @500 and 14 @1000).
So I'd go for that value of 12, unless someone has differing pen data - noting other than the wiki article came up on google with pen figures.
Andy