Vivitar T4 lens repair - unresponsive aperture

I picked up a really cheap Vivitar T4 lens today - 55-135 f3.5 - I figured it would be fun to play with on my mirrorless. I understand these lenses come with an adapter, mine is M42 mount, which has a spring loaded manual mode switch. The problem is that even when the switch is held down on M, the aperture blades do not stop down.

I had the M42 adapter off, and I was fiddling with the lens and holding it up to the light and suddenly the aperture blades seemed to close all on their own as if by magic. I fiddled with it a bit more and found a pin sticking out of the back of the lens that is what opens and closes the aperture blades. If the pin is pushed in, the blades are open, if it's allowed to stick out, the blades close. I assume this pin should be spring loaded, but mine is not - it just flaps around in the breeze and seems to open and close on its own if I give the lens a gentle shake or flip it upside down.

I am hoping somebody here is familiar with T4 lenses and knows about this problem and if it's a relatively simple fix.

I've attached a photo of the rear of the lens with the pin circled

The T4 was a universal mount that could be mounted on a wide verity of lens with an adapter reducing production costs. It was common/popular in the early 1970's. I do not know its years of production.
Does switching from I to O have any affect on the pin? (with the adapter removed)
The rear collar of the lens should come off by removing the screws in the rear of the lens barrel. Be careful lifting the collar off as something may fall out. Have the lens standing on end and gently lift straight up. The screws may be retained by a clear thread locker that can be dissolved with nail polish remover/acetone.
Take good pictures to show correct alignment, go slow and be cautious. It should not be complicated. Aperture blades should be free of oil and grease. Extra fine powdered graphite can be used on the aperture blades and pivot pins. The pin may use a trace of light weight grease. Trace-touch a cotton swab to the surface of a container of grease, wipe the surface to be coated with a light to medium pressure, wipe a second time with the unused end of the swab with the same pressure as the first wipe, what remains is a trace.

T4 lenses are shit on film. I had to shoot w/them for almost 3 yrs.--no brilliance to the glass; but might be fine for digital.

Tell us how you really feel chip.

I've had a few T4 / TX lenses, that should be spring loaded from what I remember. I don't have one now in front of me but I'm pretty sure it should be spring loaded like the pin on the back of an M42 mount lens.

Are you comfortable taking off the next layer (should be 3 flat head screws from what I can see in the photo) to see if the spring is simply out of position?