What is it

[prog]

I found this object while unpacking. I know what it is, even though I haven't had reason to use it in, oh, 16 years or so.



Do you know what it is?

(The thing on top is a plunger which presses straight down, and springs back up when released.)

Identified first by kyroraz in comments!

Comments

[dougo.livejournal.com]

That was my guess too. I just used a regular hole puncher.

[spatch]

So many 5 1/4" disks of mine with a square notch on the left and a semicircle on the right...