I found something that might work really well. Check this out, stock and customizable versions that are usable for almost all types of telephone numbering:http://www.thestampmaker.com/Departments/Rubber-Stamps/Number-Stamps.aspx
a) In-stock models: With movable bands with the numbers 0 - 9 and a blank space on each band. Six, eight, and ten band versions. Use the six-band for formats up to X XXXX, use the seven-band for format NXX XXXX.
b) Custom models: To make a six-band with a dash, for X-XXXX, or seven-band with dash for NXX-XXXX, or a twelve-band with dashes for NXX-NXX-XXXX: Click the link at the end of the line of text above the pictures on the webpage above: "If the number stamps below do not meet your needs, or you need characters other than numbers, try our LINK: Custom Band Stamps." That link will take you to a page with a "Band stamp wizard" link on it, that brings up a configuration page that works in Windows and Mac browsers.
You can probably custom-configure with three alphabetical bands, and six numeric bands, to get two letters, space, single digit, dash, four digits. Thus, numbering such as KL 5-2368. And with three alpha bands, you could spell out EXT (or four for UK "EXTN") followed by however-many digits.
Typical price for the 12-band would be about $40. Takes two weeks to arrive. Not bad, eh?
Suggestion: Laser-print your dial cards with the full exchange name spelled out (and the abbreviation Ext. or Extn. if needed), and get a custom 4-digit stamp for filling in the digits. For example:
KLondike 5 (or three-letter prefix such as MONument) (laser printed)
2368 (number stamped)
Ext. (laser printed) 123 (number stamped)