1d Penny Pink Envelope Weird Size

I have a penny pink pre paid envelope with an undated 1d pink embossed stamp.
The size of the envelope is 90mm by 150mm which does not match any of the envelope sizes of the period in Higgins & Baker. It matches type O from H&B, but that size is not listed in that time period.

There is a embossed label on the back for John Dewhurst & Sons.

Unable to see closeup of the embossed stamp,
(need to see the pendent curls)

I'd suggest it is a Brightmore #3 1885 1d Pink with both 5 or 9 florets

STO (Stamped to order)
Envelopes were in many sizes and made up from many colours of paper.
Exist : Multiples and combinations of stamp values can be be found on stationery

Strange: Google maps do not show an Oakfield Road in Lower Clapton.

Nice fancy Geometric Postmark !
Maurice Barette FRPS 1994

Not my area of expertise. If my memory serves me, it was possible to get your own stationary embossed for a period of time.

These were produced for many years.

Printed To Private Order (PTPO) would result in many different sizes.

I have a quite a number of these envelopes. Most are quite a uniform size but a few are larger.

From 1892 to 1951

I am not aware of the segregation of Post Office Envelopes printed to private order, and Personal Stationery envelopes Printed to private order.

Envelopes STO were available to 1973 according to Brightmore

"Compound stampings abound on many sizes of envelopes"

What is the best reference book. I thought is was H&B but I see references to Brightmore here.

It depends on your geographical area of interest, I suspect. This is the standard volume for France

https://www.yvert.com/A-153566-enti...-monaco.aspx

Just had another look at the back of the cover.

I have very fond memories of Skipton.

If Crispin is looking in, Bobby, he'll give you a Skipton sitrep.

Huggins and Baker is the best I have come across,
but I am having trouble reading it, after a cursory look.
Hence I go back to Brightmore for an easy ID.

I'll have to sit down with H&B and study it more intently.

Righty oh.
I have spent some time with Huggins Baker
Fabulous tome, takes some getting used to.

Henceforth I shall marry HB with Brightmore, and use Brightmore
for my references.

HB is so specific, ( envelopes with specific knife cuts ID)
I cannot use that for "Mutilated stationery"

So best route for me is a general ID (Brightmore)
added to that, a lot of Brit early stationery could include "printed paper"
Customers could have any amount of different paper embossed for use.