I don't know how I came to loose my original quick release plate, (& it still hasn't turned up anywhere), but I had to order a replacement for my 322RC2 head from somewhere.
I assumed that manfrotto quick release plates are all identical no matter which tripod head they originally come from - with the exception that some have a 3/8 fixing screw intended for larger bodied cameras, instead of the 1/4 UNC as found on the majority of camera's.
It arrived within a couple of days & was immediately fitted to the head to make sure I hadn't made a mistake about its 'universal manfrotto fit'. There it stayed until a couple of weeks later when I took it out on location to do rare steam loco' shoot. With a great location sorted along the track-side, I intended to set up the camera to shoot remotely via an intervalometer, but when I came to attach it to my Canon I found that the small 'Locator' it had projecting from its surface to the side of the mounting screw didn't actually locate anywhere on the underside of my camera & therefore prevented the camera attachment.
For the next 15 minutes, I desperately struggled to remove this 'Locator' with everything from car keys to sharp stones & curses, as I unfortunately hadn't anticipated the need to take any tools along with me on this particular shoot.
Try as I might, I couldn't get it to budge - or even work out its method of attachment, so was eventually forced to shoot the 80 mph loco handheld.
It wasn't till the next day, when armed with a sturdy vice, screwdriver, & hammer was I able to remove the little blighter!
~For anyone else wanting to remove the locator, Ive since found out you need to squeeze its 2 halves together with pliers whilst pushing it out from the underside of the mount ~
Come on Manfrotto, get your act together & please consider stupid people when designing your products !!