Are you refering to the two dumppy thing on the plug to lock it in place? If you don't have it, te plug will slide out very easily due to lack of dumppy thingy.
No, I mean the plug socket is shaped kinda like this:
----------- | | \---------/
So there is very definitely a top and a bottom. Except it's too small a difference to be immeadiately obvious and it also never seems to be consistent which way should be the top or bottom between devices.
(*sigh* why does C9 make it so f**king difficult to just stick in a quick bit of ASCII Art!)