A very nice piece with beautiful details.
On the obverse one can see the initials НФ - Nikolai Follendrof who held the office of Mincmaster of the St. Petersburg mint several times.
Obverse: the eagle of the Empire of the 1832 pattern, between its legs and tail the initials НФ, in a rope border, in the outer border an inscription beginning with a rosette:
ЧИСТAГО СЕРЕБРА 4 ЗОЛОТНИКА 21 ДОЛЯ;
Reverse: in a laurel and oak wreath crowned inscription:
РУБЛЬ/1878/СПБ;