That's an add on later style wiper arm mount or whole wiper motor assembly. The stock wiper motors have a hole and slot, the nut holding it to the truck will slide right off once loosened. I would bet you need to unscrew the center piece with the slot so the splined piece will come off. This is an old e-bay listing, yours should be like this.
NOS Trico Vacuum Windshield Wiper MotorThe side piece is a adjustable valve for speed control. I may have some pieces if you need a new shaft.
