What Kind Of Cancel Is This ?

Hi. While sorting through a collection of stamps I inherited from my grandfather, I came across this cancelled airmail one. Has anyone ever seen anything like it? How unusual are they? Anyone with anything to impart regarding this very interesting cancellation (if that indeed is what it is), I would love to hear (read) from you. Thanks in advance!

I believe it is very common. Happens to be canceled twice. I have many dozens of stamps with similar cancel.

It gives me the heebee jeebees!

It is not uncommon for a clerk finding a letter (particularly with multiple stamps) to apply one full cancel falling mostly on the envelope and then to tilt the canceller slightly and apply the killer end to the remaining stamps. Note the killer impression at the right end of the OP's stamp is lighter along the left edge and darker along the right edge, evidence of a tilt. Here are two other examples of full cancels with extra partial cancels:

But this one literally looks like eyes, a cat's eyes maybe, as if the clerk was having a little fun with the stamp. Or maybe I'm just "seeing" things???

It's definitely the same cancel applied two times. Perhaps the clerk was having fun. We'll never know.

Here's an image of a duplex canceller:

By rule, the circular town/date canel had to be placed off the stamp(s) so as to be fully readable. The stamp(s) had to be canceled otherwise with a "killer".

So with this duplex cancel, everything could be done with one strike. Of course, if there was a strip of stamps on the cover, the clerk had to find a way to cancel all stamps without the circular part obliterating any stamps. One way of doing that was, as John Becker explained, slightly tilt the canceller so that only the "killer" would ink the stamp(s).

A clerk just doing their job .....

Touche!