Like B425a, but revised signature (Chuluunbat) and a slight modification at some point between prefixes AF and AH.
On later issues (bottom), the signature ends with a curved horizontal line which is missing from earlier issues (top).
More significantly, on the back of earlier issues (top), there are bands of red at the ends of the horizontal bars along the lower edge which are not present on the later issues (bottom).
Courtesy of Alexander Petrov and Sergey Vostrikov.