The following is an excerpt from Postage Stamps, Puffin Picture Book 69 (1951) by L.N and M. Williams.

Many stamps have been forged, not only the rarities but common issues as well. All forgeries differ from the genuine stamps in some way. On this page you will see genuine stamps and forgeries side by side. Examine them and see if you can tell the differences.

A notorious forgery and how to recognise it
In 1872 and 1873 thousands of forgeries of the British is stamp were made, and were sold over the counter by a clerk at the Stock Exchange post-office in London. The spurious stamps are known as ‘Stock Exchange’ forgeries (pictured below). They can be distinguished from the genuine stamps by having no watermark in the paper; genuine examples are watermarked with a spray of rose. Besides that the whole design of the forgeries is a little blurred, and the angles of the corner squares are mostly blunted, whereas the design of the genuine specimens is sharp and clear, and the angles of the corner squares are pointed.