Like Steve said, the "K" designates where it was made. Same thing as the A - when Singer started using a letter for teh USA made machines.
Sometimes there are other differences too. From what I've gathered, Singer stopped making the 99 in the US before they stopped making it overseas, so all the later model 99's are "K" machines. By then they were exporting a lot more too, so finding European made machines started to be way more common than the older models.