To the best of my knowledge the patent referred to is Stanton's rebounding lock as exhibited in your locks. A very large number of British rebounding locks from this period carry the 'joint patent' stamp.
I have several theories about this (someone may know the facts - I can only guess).
1. Stanton allowed the use of his patent for a reduced fee if the stamp was used. It was common practise to lay claim to a current patent by insisting the component was stamped with the use details, maybe this was Stanton finding a compromise to deter fraudulent use of his patent is an area rarely seen by Joe Public.
2. Perhaps, like Westley Richards with the Deeley ejector box, Stanton insisted on the locks being returned to him for inspection before he would allow the patent use and he then stamped the 'joint patent' mark to authorise the job.
3. The stamp was used when a non-rebounding lock was converted to rebounding (very common practise when the patent was first unveiled) but somehow I doubt it as too many of these 'conversions' are rather too late to have been built as non-re-bounders.
