As instruments became standardised, they had to be manufactured and/or tuned to similar frequencies so that they would play in tune. It appears that in the 19th century most brass instruments played at a pitch not far from the current standard of A = 440 Hz. Levels of pitch rose during the century generally, with brass bands keeping in line with other instrumental and orchestral ensembles, arriving at the “Old Philharmonic” pitch of A = 452.5 Hz. Towards the end of the century pitch standards dropped again for orchestras, followed later by military bands. The latter adopted A = 439 Hz in 1929, and the former finally fixed on the international standard A = 440 Hz in 1939, led by Germany. It has to be said that it took many more years to get full international agreement, and the occasional country, ensemble or instrument were still at variance with the standard (either significantly up or down) in the early 1970s.
Brass bands, however, remained in high pitch, around 453 Hz until the early 1960s when there was a gradual conversion to low (standard) pitch. The decision by the brass instrument manufacturers to cease production of high pitch instruments (e.g. Besson in March 1965) accelerated this trend. The makers followed these decisions with the provision of tube and slide extensions to enable the conversion of existing high pitch instruments to low pitch. A by-product of this change was to open up the British brass band market to foreign instrument companies, who quickly started to make inroads into the brass band scene.
The full story of pitch changes, standards and the conversion of instruments is complex and includes many changes of direction and compromises. I am not aware of a “standard text” on the subject, but it is partially covered and referenced in many publications and online resources.
In 1969 Boosey and Hawkes were still trying to persuade bands to convert (hopefully by buying new instruments) – though their advert of that year might raise a few eyebrows today!