Here is a batch of overdate coins that I bought over the last few days, if not weeks. I'm losing track of time

First is the 1821/0 poltina


Getting a bit difficult to find but I'm hoping to find the 1821/0 ruble to pair them up.

And here are two that I recently bought from Blackhawk - I'm quite puzzled over the overdates as these are struck a lot later.

First one is a 1878 20 kopek:


At closer look, it seems that the 8 is overdated over 7


Another one is this 1880 20 kopek:


At closer look, this too seems that the 8 is overdated over 7 and the 0 might be overdate on another digit, could be a 7.


I grabbed a look at Severin and it seems the mintage for 1877 20 kopek is one of the lowest. Do you see the overdate as I do and what is going on? I thought overdates would have pretty much died out by then. Also, what is the last known overdated coin for Imperial Russian coinage?

I know nothing about the 1878 but overdates for 1880 (10, 15, 20 kopecks) have
been published. In the case of the 1880 the ones I have seen are over a die with
187–, meaning that the final digit had not been added. This gives the appearance
of 1880/1870 but is really 1880/187–.

RWJ