GB Scott #5 (Sg #55?) Identification Question

I recently won an auction lot of early GB issues. One of the items in it was identified as Scott #5, but the paper makes it feel more like a cut square from a postcard. There is a dark vertical line running down the centre back of the stamp, which could be the "embossed silk threads" mentioned in Scott.

Does anyone have a reference copy of this stamp who can tell me if it was printed on an unusually thick paper, or if it is likely a cut square that I have? With a difference in catalogue value of $900 for the stamp and probably a few dollars for the cut square, it would be nice to know.

This item wasn't the reason I bought the lot; in fact, #5 wasn't even a part of the description. If it turns out it was mis-described, I'll keep it as an old cut square.

Thanks in advance for your assistance.

Out of my comfort zone, replying as practice.
Opinion.
Has to be a stamp, as Sc#5 Octagonal shaped print?
Cannot be stationery, 1841 stationery were Oval Impressions
If yours is "cut to shape" that is, cut to the octagonal shape of the stamp image itself, then that attracts the lower price .
Happy to be corrected.

KD, can you show us an image of the front and back.

Just to clarify for anyone wondering, this is the one shilling value.

There's no mention in the QV specialised of varying paper thickness.

There should be two vertical silk threads (they're actually twisted cotton yarn, blue in colour) in each stamp, 5mm apart.

The one shilling die used for stamped to order envelopes shows the date of printing in three circles at the southwest, south and southeast of the inner octagon.

Thanks Bobby,
that clears that up for me, so dated STO (stamped to order) were from 1st October 1855, the die impressions (dated) on the STO envelopes, could be cut from any envelope and used as adhesive stamps.

Here is my Great Britain, Scott #5, from my collection, and the paper is relatively thin, not thick at all. You should be able to see the two parallel threads in the second scan, one running vertically touching the Queen's nose, and the other one 5 mm to the right.

Hope this helps you,
Linus

Postmark: Possibly 302 = Saltcoats Ayrshire Scotland.
wiki
Saltcoats is a small town on the west coast of North Ayrshire, Scotland. The name is derived from the town's earliest industry when salt was harvested from the sea water of the Firth of Clyde, carried out in small cottages along the shore. It is part of the 'Three Towns' conurbation along with Ardrossan and Stevenston.