TKL is basically the same as a laptop keyboard, which up until about 10-15 years ago always had a numpad overlaid on top of the letter keys below the 7, 8, & 9 keys and activated by pressing the NumLoc key. I'm guessing manufacturers stopped including the feature due to more and more non-technical users using laptops, and confusing themselves by accidentally turning on the NumLoc (business-oriented laptops often still have them). The image I linked to above is from a page explaining it on a website related to the once-popular "For Dummies" series of books & they have an example of what happens when you type with it off vs on.