I was looking around for info on this, and found this thread. The original poster is the late, lamented Al Kaplan, sigh. He passed away near the end of 2009--I wonder if he ever got his catch tightened up.
Anyway, for future reference to anyone in a similar situation, I'd like to add my bit, at least relevant to an M3. Mine also suddenly became incapable of staying folded flush to the base plate, and kept flopping down.
I found that neither tightening the screw on the interior of the base plate nor putting a little more bend in the dish spring washer solved the problem--the apparent purpose of the washer is only to provide a little resistance to turning the locking plate, so it doesn't come unlatched accidentally.
It appears that the somewhat U-shaped external locking/unlocking tab is held in place against the base plate only by friction between its inner edges and the hub to which it is attached. I was able to repair mine by removing the central screw and taking out the hub and tab, then pushing out the metal pin that secures the tab to the hub.
*Very* carefully using a good pair of channel lock pliers (vise grips would work just as well), I was able to squeeze the two ends of the tab, where the pin inserts, slightly closer together. It is extremely important to go slowly with this--it did not take much movement at all to squeeze the ends sufficiently to solve the problem. And it is much easier to push them closer together than I am certain it would be to spread them back apart should you go too far.
It would be a good idea to do this on a table, on top of a white sheet or towel. The central screw is pretty small and painted black, and there is a tiny black washer just beneath it that if dropped would be very difficult to find. The towel would not only make things more visible, should you drop a part, but the fibres should help catch stray bits and keep them from bouncing away. A magnetic screwdriver also would speed the process.
It would also be helpful to have a second pair of hands around. There is a square projection at the top of the hub which fits into a similarly shaped hole on the dish spring washer to make sure they are aligned properly. It doesn't stick out very far, so it was much easier to finish the repair when I held the spring washer to the top of the hub while a friend maneuvered the small round washer and the screw into position and tightened it to wrap things up.
Mine now works fine--it was about a 10-minute job, but that was only because it took us figuring out what we needed to do. If I had to do it again--and had some help--it would take half that or less.
