I have RRS L-brackets on both my D700 and D800. I just looked at them and they are somewhat different. One place in particular is the back of the bottom piece where it snugs up to the camera body. They are shaped different in this area. Since the D800 L-bracket fits like a glove in that area, I don't think I would be comfortable tightening the D700 bracket onto the D800 body, or vice-versa. I guess it could physically be put on there, however it may put pressure in places that are not meant to have pressure, and it may hinder access in areas.
Bottom line, mine are different, and I would NOT want to switch them.