I know with this particular lens, you could take apart enough of the lens to get to the other side of that tiny pin. That's not always an option but Takumars are built to be disassembled that way. You'd have to unscrew the trim ring on the front and remove the three screws holding the filter ring on. Take that filter ring off. Then focus to infinity and remove the three screws holding the focus ring on, and remove that. Then loosen the 3 tiny slotted grub screws on the side of the lens holding the DOF scale ring on, and slide that off. Now the aperture ring can be slid off, but don't lose the bearing trapped between the ring and lens body. It's just above the orange diamond.
Under the aperture ring are three screws holding the lens mount on. When you remove those, the rest of the lens can be removed from the mount. Now you can get to the other side of that pin, and pull it up enough to unscrew the mount from the adapter.
The 135/3.5 is just about the same as the lens pictured in
this disassembly guide, except longer. You only need to go to about step 8.