Hasselblad Magazine 12 (C12) Film Back Light Plug

Hasselblad's original Magazine 12 film back (1947 - 1970), also known as the C12 film back, has a hole at the rear so that you can see the numbers on the backing paper of 120 film, to align it with the first frame. To use this back with 220 film, you would cover the hole with a supplied plug, to prevent exposing 220 film which doesn't have backing paper.

If you have a C12 back, you probably don't have the light plug. Which is fine since 220 film isn't available.

But if you want to use one