Katie,
had you eaten your Wheaties before you pulled on the switch?
You have to really pull hart and straight out to get it off.
These pin holes are designed for a plier to turn/unscrew the collar/nut. Mercedes had a special tool that was never sold to the public but provided to its company workshops. I have seen a photo once but cannot find it anymore.
I have used really cheap snap ring pliers ($5 at Harbor Freight), heated the pin ends with a flame until red and then turned the ends so that the tips face each other.
I tried the straight ones in the middle first to practice bending when red hot and then used the ones that are angled as shown in the picture on the left.
All 230SL had the collar with the 4 pin holes (now NLA), 280SL have the slotted version (don't know what is on the 250SLs).
I doubt you will be able rescue the one your car, thus you might as well use vise grips to get it off.
Go with the slotted version (Product No.: 254043:
https://www.sls-hh-shop.de/main/en/mercedes-230-280sl-w113-/54-electrical-equipment-at-body/54-c-equipment-for-cable-harness) unless you insist on originality. If so, be prepared to pay $30 + shipping for a good used one.
When you have the switch out note how the sheet metal is bent inward (towards front of the car) with two cut-out notches. There are corresponding protrusions on the switch that fit into the notches and lock the switch in place providing the correct horizontal orientation of the knob. {Not sure whether the 280SL dashboard metal is similar}.