What do the 'X' and 'K' codes mean for rubber bands? I see that LEGO rubber bands/gaskets are often associated with a number preceded by an 'x,' such as 'x71' for the small/18 mm bands. A few bands also have a 'K' prefix, such as 'K4' for the large/30 mm units. This latter item is also known as and 'x89' rubber band.

I'd like to know what these prefixes mean and where they came from. Likewise, I'm curious what the actual numbers refer to.
The prefix 'x' means that the element design-ID used by LEGO is not known or was not known at the time it was entered into the Peeron database (www.Peeron.com).
If a design-ID is known, the people who maintain Peeron use that to identify the piece. Otherwise you get 'x' with some number...
This is indeed often the case with strings, rubber bands, gaskets as there is no room to print the design ID on the element.

I have no idea about your 'K' prefix, could you give a reference where 'x89' is refered to as K4 ?