Four paintings have been located that may very well have been painted for the exhibition but were apparently excluded for unknown reasons.  

The first, titled Ice-Field Off "Newfie", is in the collection of the Vancouver Maritime Museum and came from the same donor as three confirmed exhibition paintings.  It is numbered with a circled #4 and titled in the same manner as most others.   Winter sea  ice was a frequent threat to the convoys and escorts off the east coast of Newfoundland near St. John's.  The ship depicted is a corvette with its 4 inch gun in the foreground.  This picture undoubtedly fits thematically.

The second is titled Moonlight and numbered #18 on the reverse.  Riordon clearly painted two identically numbered versions of the same subject but this version was not chosen for the initial Montreal exhibition.  Possibly it appeared at one of the subsequent venues but there is no way to know for certain.  Both  versions of "Moonlight" match the description in Riordon's narrative.

The third picture is apparently titled "Last Moments Of The Etchett", a title which bears no relation to the subject and is likely a misattribution.  U-boats did not have English names and an extensive search has not turned up any sea-going vessels with that name. The picture does have characteristics that suggest it could have been painted for the North Atlantic Convoy exhibition.  The dimensions are 6" x 8" and the subject matter is of a battle between a corvette and a surfaced and damaged U-boat.  A pair of approaching airplanes are identifiable as long range Liberator bombers with their high powered Leigh Lights illuminating the attack.   The description of one of the missing pictures, #33, somewhat resembles this picture.

The fourth possible candidate for the exhibition is the picture variously identified as "Canadian Tribal Destroyer - In Convoy" and "Sunset - Destroyer".  A reference appears on the Internet suggesting it was sold at auction around 2011.  It appears to have the correct 6" x 8" dimensions and seems to match the description of either #7 or #16 in the convoy series.

Riordon definitely painted multiple versions of some exhibition pictures.  The person who organised the initial 1950 exhibition, presumably Mollie, made her choices as she saw fit.  It is conceivable some of these "extras" or doubles were exhibited at subsequent venues.