That rail is held in by two pins so it's not going to fall off if the loose pin is removed and replaced.
What I think has happened, though, is that the pin has broken leaving the head in the hole in the block on the inside of the rail. I found an old pin from my 110 engine last night, and there is a groove about 5mm from the tip that would be a primo place for the break to occur. I think that the inside hole provides the real holding power for the pin.
Why don't you remove the pin for inspection and see if it has broken?
I am unsure of what advice to give you if this has happened. One difficult approach would be to try to drill the piece, insert an easy-out, and try to attach some sort of puller. Another would be to roughen or otherwise treat the surface of a new shortened pin, and install it with loctite and hope that the outer hole will hold it.
I guess I would try the second, on the theory that you could always try the first if it didn't work.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe