This letterbox is located close to 'Rivercity: Over the River'.
Step off the Foot Bridge across Saskatchewan River and go forward, towards High Level Bridge. Very soon on your right the road you are on will pass close to the bridge support columns.
Take a break - sit on the front, closest to the road, cement base. When no one is around, go to the back of the column. There above your head (be careful of those metal railings)the letterbox sits. Can you spot it?