Broken-coin, some nice examples you got there. I don't think I've seen a clipped planchet of that error and this could be a lot scarcer than what you may imagine. I also like the double struck denga.
I personally do not classify overstruck coins as errors. Reason being is that as mints during those era intentionally used older planchets to either cut cost or decided to skip the process of destroying older coins and strike new coins due to various reasons. I've written an article a while back which you may find of interest: http://gxseries.com/numis/overstrik...erstrike.htm
I'll attempt to solve the mystery that you have with the overstruck coins. For more information about Russian overstruck coins, this may be of interest: http://gxseries.com/numis/rus_imper...erstruck.htm
For the 1731 denga, I believe it's overstruck over this particular type.

Overstruck over Peter I 1710s kopek coinage. Note that there's absurd amount of varieties for his coinage.
The 1757 2 kopek is overstruck over Baroque kopek. This is easy to tell as you can see the eagle features as well as the cloud feature

I personally do not classify overstruck coins as errors. Reason being is that as mints during those era intentionally used older planchets to either cut cost or decided to skip the process of destroying older coins and strike new coins due to various reasons. I've written an article a while back which you may find of interest: http://gxseries.com/numis/overstrik...erstrike.htm
I'll attempt to solve the mystery that you have with the overstruck coins. For more information about Russian overstruck coins, this may be of interest: http://gxseries.com/numis/rus_imper...erstruck.htm
For the 1731 denga, I believe it's overstruck over this particular type.

Overstruck over Peter I 1710s kopek coinage. Note that there's absurd amount of varieties for his coinage.
The 1757 2 kopek is overstruck over Baroque kopek. This is easy to tell as you can see the eagle features as well as the cloud feature

My partial coin collection http://www.omnicoin.com/collection/gxseries
My numismatics articles and collection: http://www.gxseries.com/numis/numis_index.htm
Regularly updated at least once a month.
My numismatics articles and collection: http://www.gxseries.com/numis/numis_index.htm
Regularly updated at least once a month.


















































