"Tatham Stamp Company
If your KGV Green block is genuine, can fetch a good price apparently."
(I only assume it is, but you never know)
Tatham Stamp and Coin seems to come up as a popular topic every so often on the internet and eBay
. It had a pretty colorful history and products.
This unused return envelopes backside ad offer some interesting deals.
I guess somewhere between 1952-58 as it asks for 3 cent return postage.