Hi perfectdaze. Another good photo of the Prom' I notice that the postcard has the description - Number and the company name at the bottom. I've had a rummage for info on this company.

http://viewfinder.historicengland.org.uk/story/intro.aspx?storyUid=77

You may want to have a look at this webpage - if you do - scroll forward screen 13 to see a postcard of Southport's Opera House. I hope it works for you!!