Hi Jamjar,
I have asked around and the most likely site of the old Commercial Inn is considered to be where the currently vacant former chemist/newsagent is today, located opposite the Tower Fish bar at the Cross on Lockerbie's High Street.
I have attached two images. The first is a photograph of the building taken in 1890. The sketch drawing is of the same site from pre 1848. In this sketch, I believe I can identify the word "Inn" on the signage board and the first word could well be commercial.
See what you think...
Ian Henderson. the local solicitor who has retired I think, but still has an office on the High Street has the original painting in the upstairs rooms, and he may well be able to assist you. If you were to contact their office, they may be able to assist. On your behalf, I have telephoned their office to see if the paintings are still there, and they are!
The Ewart Library may well have the same images in their archives. It's definitely worth a look.
Hope you find it.
Jak
