Reciprocity Timer 1.0

[BradS]

So then what ARE they for??

The bumps are there so that when you''re in total darkness loading film holders, you know how to insert the dark slide so that the 'right side' is facing out. Most people let the bumps face out (white band out on modern Fidelity and Lisco) to indicate "loaded and un-exposed". After exposure, you flip the dark slide over to indicate "Loaded, exposed".