great find Roy, well spotted - and good pix by the way.
I notice that Charles Hajdamach omits this item from the Jobling pieces in his book, as does Roger Dodsworth in his 'British Glass between the wars'. Sheilagh Murray appears also to have been unable to source a picture of this item, so looks like you're right about it being scarce.
In view of your comments about the lettering on this ashtray, it would seem that this design was put into production prior to the factory being allocated a Board of Trade Rd. No., although as can be seen in Baker & Crowe, a number was provided PRESUMABLY after production had commenced.
It would be reasonable to assume therefore that either......your piece is an early item and later mice were provided with the Rd. No., OR PERHAPS, the moulds were never changed to incorporate No. 796187 ?? Might Stephen have thoughts on this??
Hope that Stephen and yourself you won't object to me adding the Registration details which may prove to be useful re the Boards search archive.....
Registration No. 796187 - dated 6th September 1934 (expired 06.09.1939) - Jobling Cat. No. 10510
Like a number of other models from this factory, the original plaster design for the mouse ashtray is credited as having been made by Franckhauser.