I own some bottom loading Zorki. I also have an ABLON copy. I use a tool made for ladies when sewing to cut the leader. It is used when one has made a wrong sewn and want to cut it. It looks like a little hook with an handle. Even in the dark you won't hurt yourself if mishandled.
Have a look here : http://www.aiguilles-magiques.com/musee/decoudvite.htm (the picts are not terrible but you'll see what to ask for !)
Hope this helps !