Vivitar 135 2.8 Switch. What does it do?

I have this Vivitar 135 2.8 in M42 mount that I got in a yard sale, and it has a little wheel on the base that has an arrow and a red 'M'. Macro? I forgot to take a pic of it, but the other mystery that I DID take some pics of is: if I push the sliding switch on the other side of the lens base which says 0 in red on one side, and L on the other side, the whole back of the lens pops out? What is going on here? I've never seen a lens like this.

The red M is probably the auto/manual aperture stopdown switch. Stick it on your camera and play with apertures to see if that changes anything.
The O(pen) / L(ock) is for swapping the mount, look like it's got a T2 or whatever-style interchangeable mount.

Yep, when you turn the wheel to the red 'M' it stops the lens down to your preset aperture. Thanks!

So this is one of them T mount lenses. I have never seen one before. I suppose you could buy different mounts for the lens and just click it in, then shoot it on other cameras? Neat. When I first pushed the switch and the whole end of the lens popped off, my first thought was... Oh no, I broke it.

Not sure that this is a lens that would be desirable enough to warrant that feature, but you never know till you shoot it I guess. It's sure heavy, and has a big old front element. Thanks again folks.

Shoot some with it. Some Vivitar lenses have great reputations. I don't recall which ones so you'll have to use it.