Good looking coin in pretty good condition, the ID from Dad is:
From the legend elements visible – my view is most likely a Gratian AE4 of AD378-383 Sears(88) 4145 or 4146.
The obverse I believe is: [DN GRATIA]-NVS PF AV[G]
Reverse could be: VOT XV MVLT XX (4145) but I believe is VOT XX MVLT[X]XX (4146) Mint not clear.
Hi Cru, great ID from Crudad again. Wasn't Gratian not an usurper from Britannia?