While ciphers used during the English Civil War were mostly figure ciphers (see another article), Glamorgan's cipher was based on line segments with varying length, positions, and orientations. It is not unlike a cipher used by a Peruvian official in 1604 (see another article).
The Earl of Glamorgan (later Marquis of Worcester) was authorized by King Charles I to make concessions to the Irish Catholics, which would not have been approved by the Marquis of Ormonde, the Protestant Lord Lieutenant of Ireland. Indeed, the treaty Glamorgan made with the Irish in 1645 drew such a fierce criticism that the King was compelled to repudiate it.
The following letter entirely in cipher preserved among Glamorgan's papers was identified by Dircks (1865) to be a letter of 29 September 1645, from Glamorgan to Ormonde. Besides the textual differences from the known text of the letter pointed out by Dircks, the word "discrimination" appears to be actually "diminution."
Actually, before Dircks, Rees' Cyclopaedia printed the same cipher, even including the cipher for "z", which does not appear in this letter.
William Blair, the author of the relevant article in Rees' Cyclopaedia, saw this in Biographia Britannica, vol. 1 under article "Bales" (p.540 in the 1778 edition). It is the same cipher as that used between Charles I and Glamorgan (see another article).
According to tonybaloney, the key written by the King himself is reproduced in Pall Mall Magazine (1896) Vol.8 No. 34 p.256. On the other hand, Glamorgan called this "myne owne characters" when he used it for hiding his knowledge of which he feared ill use.
The Marquis of Worcester's Century of Inventions catalogs a hundred inventions claimed to be made by him. While many items relate to cipher or communication, his descriptions are very brief and commentators, including Dircks, had to make speculations as to their substance. Probably, Dircks is right in saying cipher alphabets are the least interesting portion of the "Century" (Dircks p.436).
At least one specific scheme is explained in manuscript papers (discovered by William Blair). It enciphers the 24 letters of the alphabet by lines which may take three different lengths and eight different directions (Dircks p.394 ff.).
Henry Dircks (1865), The Life, Times and Scientific Labours of the Second Marquis of Worcester, p.180, 394, 553 (Internet Archive)
Rees' Cyclopaedia (1802-1820), (Search Internet Archive, Vol. VIII (1807), Plates Vol. IV (1820)).