Corieltauvi 'DVMNOC TIGIR SENO' Silver Unit. AD 15 - 20. 17mm. 

As Van Arsdell comments, the obverse die appears to have been used for some time before the boxed inscription was added, as the wreath is very worn, but the inscription is clear and sharp.
It has been suggested that the obverse inscription refers to a primary ruler, Dumnoco, and the reverse to a secondary ruler: Tigir Seno.

Obv: DVMNOC, in rectangular panel, Apollo-wreath divided by inscription.
Rev: TIGIR SEN[O], above and below Celticised horse right.

VA 974-1; Mack 462; Spink 415

Courtesy of the finder, metal-detectorist Steve Green, Nov. 2010.