The lettering on this one looks suspicious; if it's genuine, however, it looks like an overdate:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewI...em=160217323093

What do you think?

51 from 49? maybe...

Never mind there was no SPB-NG rouble in 1851, my bad (no catalog on hand)

The dies seem normal for this period, the obverse having several small breaks along
the rim. It appears to me to be an overdate, 1841/183–, meaning that the die was probably
made in 1838 without the last digit. It was not used in 1839 due to the very low mintage
of regular issue roubles. For some reason it was also not used in 1840 and wound up
being dated and used in 1841.

I have seen this overdate before and it seems likely that a considerable number of dies were
prepared in 1838 but not used for two or three years.

RWJ