The 21st Lancers (Empress of India's) was a cavalry regiment of the British Army, most famous for its participation in the Battle of Omdurman, where Winston Churchill rode with the unit as a reporter.
The Queen's Royal Lancers (The 'Death or Glory Boys') is an armoured regiment of the British Army.
This comes from the 17th/21st Lancers, and was the cap badge of the 17th Lancers (the original 'Death or Glory Boys') A cap badge is a badge worn on the front of uniform headgear and distinguishes the wearers organisation.
In front of the cavalry on the right, he posted the lancers under the command of Protomachus, and the Paeonians under that of Aristo; and of the infantry, the archers under the direction of Antiochus, and the Agrianians under that of Attalus.
This is not surprising given its long uninterrupted history stretching from 1759 to the merger with the 21st in 1922.
The 21st has been brought together and disbanded on numerous occasions over this same period of time.
As long as you know that the author is a very sympathetic writer about his subject material, this book would be a valuable addition to an enthusiast of regimental histories.