Very often retrograde letters like on your coin indicate the possibility of "irregular" mints
but after looking through the reference books I guess your coin is Imperial.
I have found a similar coin from year 15 sold through
CNG some time ago:
I have also checked the fotos in the
standard reference works and found one moreÊ similar coin with retrograde NN
It is Tolstoj 437, also struck in year 15
Unfortunately I do not read Russian and thus cannot tell you what he writes about this coin.
Kind regards
byzantiumcoins
Foto from
CNG, 152. Electronic
auction, lot 242