Richard Freeman kindly sent me this photo of a plaque he recently purchased
It depicts Locomotion No.1 and the Bulmer Stone. The Head of Steam museum has the mould for these which was made by Stephen Bell in 1909 with the plaque being registered as a medal design in 1911.
They appear to have been sold by the Pawnbroker,  Arthur E Berry of 58 – 61 Northgate
Where is bulmers stone now, it used to be at the library. In darnton town there is a stone ….
http://www.hidden-teesside.co.uk/2012/04/27/bulmer-stone-darlington/
Thanks for that link. We have just bought one of these plaques, so great to know more about it.