[1969_CM2590D]

FWIW, here is what the the 1970 (the year of the Jimmy's debut) GMC service manual supplement states under "General Information":

The Jimmy Models use the conventional cab, but without the roof, rear panel, door glass, ventipanes, or door upper frames. This unit is integrated with a Fleetside pickup box which is shortened at the front, but without the front panel, but complete with floor side panels, and tail gate. The box side panels are welded to the door pillar structures, and the pickup box steel floor to the cab floor, to form a one-piece integrated body structure.