This is an early Indian punchmarked silver coin, and is no.506 in
Gupta and Hardaker's 'Ancient Indian silver punchmarked coins'. It is
part of their series Va, and is listed as very common.
These coins appear on blanks which can be anything from square to circular, and which just have 5 punches on one
side, often with single punches on the other
side, which are said to be banker's marks. If you have 20 of them it is likely that you have several different
types, but the proably all date from c.300-150 BC like this one.
This page has some details about these coins, and illustrates some other
types:
http://www.grifterrec.com/coins/india/ancientindia.html
