Can you identify this?

I've just got my first ever Atari 800 and the very first thing I did was to take it apart (for cleaning + curiosity). While I was disassembling it a plastic piece fell off, I have no clue where it came from but it looks like some sort of spacer, perhaps holding the RAM and OS cards together. I can not however figure how/where exactly it should be fitted, so any help is appreciated.

Thanks

EDIT: Well, doing a forum search sort of confirms my suspicion, it seems to be the spacer for the cards and it's supposed to go across the top of them. One question still remains, does the exact position matter (i.e. dead center, right side, left side etc....)