The Wrecto Candace and the Calgary group saw was parked near the location of a local convention, though it may have been parked there temporarily between filming blocks.
I asked her about the mesh surrounding the seat-mounted Pack, whether the holes were hexagonal or circular, and she conferred with one of her colleagues. To quote her reply exactly:
Full on circles
I saw the photo myself
Then there was something I came across on the Ghostbusters Spoilers Group, regarding this photo:
-A member of the Spoiler group noted it as being present at 102 N. Division St. in Guthrie, Oklahoma, and checking Google maps, the location matches 100%:
The wooden utility pole with the poster for Elaine is in the left-hand screenshot.
A different member of the Spoiler group indicated the account may've been set up by a member of one of the Oklahoma Ghostbusters franchieses. Given the new information from Candace, and the fact the street photo was taken in Oklahoma rather than Canada, I think we can stick a fork in the "Summerville County Ghostbusters" and call it: myth busted.