Hmm, I have to take pictures when I get home, but on mine it's simple; there is a black metal bit on the extension table, it fits in a metal part in the lid, and there's a tiny lever you turn in place to secure it. The extension goes in with the top side towards the side of the bent wood cover. My 99K dates to 1934, I think it differ slighly to the earlier version I've seen, where the placement of the knee lever and loose parts are organised a bit differently. Mine is very similar to
this 201.