[Ron Wood]

HerbZ,
As I have noted previously, Kl. can also represent "Klein", or "small", which is probably a bit more likely than "Klasse" as the kit is clearly designed for field repair rather than classroom instruction. Klasse, as noted in your Langensheidt dictionary, is closer to "classification" rather than "classroom". To expand a little on my previous post, the "WK." probably is an abbreviation for "Werkzeug Kiste", or "tool kit". So the marking probably identifies this as a "small tool kit", as opposed to "large tool kit", which was quite a bit bigger than this kit and contained a lot more tools for field repair of several types of weapons.