that look of haunted despair

 There are no listings for "Serviss" in the contemporary Directories. We do however find an Earl Service working as a mechanic at the Beamish Garage on Nepean Street.

The Mosgrove address was a tiny wooden house on the NW corner of Besserer, across the street from the Hop Sam Chinese laundry. It was demolished no later than 1932 and was eventually subsumed by the Rideau Centre. The north end of Mosgrove would become the Freiman Mall.

Re Denman's Drug Store — "DENMAN & MUNRO LIMITED, Ernest C Denman President. George R Munro Secretary-Treasurer, Druggists, Main store 180 Sparks, Branches 271 Dalhousie and 239½ Elgin" — 1923 City Directory.

As to the product being advertised, it was a powerful laxative, going by the list of active ingredients. Also, it says "laxative" right on the bottle.