Don’t forget that Westley Richards were at the heart of the Birmingham
Gun trade, and one of the major makers. They certainly did make Cape Guns and sidelocks as a boxlocks and detacheable locks.

A good article from a few years ago from their Explora Magazine https://www.westleyrichards.com/theexplora/westley-richards-12g-cape-gun/

And if the gun in question is in nice gun at the right price does it really matter who made it? These things don’t often come up for sale.