I just took one of these apart recently. You need to completely remove the two grub screws on the distance scale ring, and then THROUGH those screw holes loosen the three grub screws that hold the knurled focus ring to the front lens element. When those three screws are loose enough, you can lift the knurled ring off, and then unscrew the front element all the way. Take note of where the front element comes off as there are a multiple thread paths that it can go back on. Only one is correct. There are little painted indentations where the grub screws from the knurled ring sit on the front lens element ring. Check it at infinity with the split-screen on the ground glass before you put the knurled ring back on. With the knurled ring back on, the lens should "stop" at infinity. Good luck!
EDIT after seeing Richard's post above, I believe that the front group in a Tessar formula is a single element, not a cemented pair, so it shouldn't be lens separation. I could be wrong as well however.