Treasure is an Unknown postcard, Plymouth

This postcard was found in my grandmother’s things.  It has a carefully placed pin or tack holes at the centre top and bottom, leading me to believe it was pinned up at some time.  Grandma never went to England.  And some quick research on phone numbers in Plymouth leads me to believe this postcard predates her birth.  Some more admittedly hopeful elimination and I believe this postcard belonged to my mysterious great-grandmother.  Is this yet another clue to her secretive past?

Are there any postcard afficiandos out there who can perhaps tell me any more about this postcard?  I wonder how common it was in the early 1900s to have postcards made up of restaurants/businesses.  Can the interior of the restaurant tell me anything about what sort of restaurant it was, what clientelle it may’ve attracted?  What ever happened to Jones’ Restaurant at 3 Union Street, Plymouth?  Would the “85” (or “B5”?) in the corner itself be simply a photographers mark on the negative, or does it have other meanings?  Perhaps it’s just me being optimistic, but is that a man sitting in the back booth at the far right side?  Other than the telephone number, are there any other things that might point to this postcard dating to the 1905-1908 time period, or any other time period?

A simple postcard, so many questions.