Does Anyone Have Information On This Stamp?

I can't seem to find any information online about this stamp. If you can help, that would be great. Thank you!

It isn't a stamp - it's been cut to shape from postal stationery (envelopes or postcards with preprinted images).

Octagonal cutout from an envelope, first issued 1918.

Oh, thank you! That's why I couldn't find it, haha. My brother and I have inherited quite a large stamp collection from our Aunty. So, we're not experts in this field. #128516;Thanks again for your help.

Mutilated Stationery
"Cut to shape" King George V, Star Embossed Envelope piece.
Mid-Brown
ACSC #EP20 c15th March 1919 (Envelopes Page 178)
Envelopes selling price 3 for 5d

Quantities : 3,357,280 Envelopes
In several dies and Knives (Envelope shape)
Full envelope required to correctly identify

$100 mint $20 used
Cut to square c$2-$3

Mr. Robin Linke (Western Australia) lists all these dies / knives
"cutouts" at $5 each (2013)

I would be presuming that price for a "cut square" not, cut to shape.

Generally speaking (99.5% of the time), a stamp in the traditional sense will be square, rectangular, or triangular, for the obvious reasons of ease of printing, use, and transport. Any old-looking, oddball shape will almost certainly be a cutout, as previously mentioned.

Thanks very much for that detailed information.