This is the map from the book The Religious census of 1851 : A Calendar of the returns relating to Wales, Vol II, North Wales. Ed. by Ieuan Gwynedd Jones, UWP, 1981.
It shows the Registration Districts of Wales in 1851, the legend below that has the key to each numbered RD.
NB - the county boundaries as shown here are marked with a solid line, those for the RD's have a broken line - BUT some RDs overlap counties which means the county areas as shown are not strictly accurate
The book is is in 2 volumes and is based on a north/south division with the area north of CGN and RAD being in the North Wales Division.
The South Wales Division , and MON, is included in The Religious census of 1851 : A Calendar of the returns relating to Wales, Vol 1, South Wales. Ed. by I.G Jones, & D. Williams. UWP, Cardiff, 1976.