KC, that is because the later model M's weren't made by IBM themselves. Originally IBM made all of them, but then they started having Lexmark make them in 1991 (IBM continued making in their Scotland plant for a few years after they shut down their US plants though). The cost cutting measures kicked in with Lexmark. If you have a board made by IBM themselves, then you should be good.
I've had a few of the Lexmark boards, and they're not bad. I mean, I do notice a difference on my silver label compared to the Lexmarks, but it isn't a huge difference. Side by side, the average person wouldn't be able to tell them apart, and it is really only the hardcore that can tell.
When I've taken M's apart before, I did have one that didn't click in properly, but I was able to press hard to get them to pop back in (make sure you do it all the way across), or adjust by taking apart again. Check the clips, see if any are broken or bent. That may be the issue.