wd1040: very good! I have never seen anything like this before - it still doesn't matter what the original planchet is but this is the first time I've seen one that's overstruck. It's common with copper coins as Korean 5 fun were used to overstrike but this is a first.
svslav: Nick pick up. It's struck in Gansu province but this province is poorer than other regions. And such, coins struck in the area were struck in a lower silver content and if I am not mistaken, it was traded with a lower face value. Here's more information about this coin:
http://home.netvigator.com/~ykleungn/yuansk2.htm
My partial coin collection
http://www.omnicoin.com/collection/gxseriesMy numismatics articles and collection:
http://www.gxseries.com/numis/numis_index.htmRegularly updated at least once a month.