ABCOO is not for use with the ABLON, but for cutting the film in the camera. A Leica uses films with 36 exposures. For a lot of people that is way too much. For those people Leitz invented the ABCOO. Say, you've exposed 14 frames and you want to see the results, but you don't want to spoil the 22 unexposed frames that are still on your film. Then you take your Leica and your ABCOO to your darkroom, you remove the bottomplate and you position without removing the film your ABCOO next to the take-up spool. The ABCOO is very sharp, so you push it in and wile doing so you cut the film - just after your last exposed frame - in two pieces. You now can take out the take-up spool and develop the piece of film with the 14 exposures on it. That is what the ABCOO was made for. I hope this is clear to you as my English is not very good and the procedure of using the ABCOO is quite complicated.
Erik.
