"Incredibly expensive to alter (correct, update?) the pages of the books...", for a company that made millions of pounds per year selling this stuff?
BSA was doing so badly by 1969 they were broke. In 1969 BSA was begging for financing from "City" investors and being turned down. By 1969 the City had given up on investing in British Industry. Then in 1969 along came Peter Thornton in the USA. His expenditures on the Triple Racing program, his life style and warranty costs were draining millions of pounds a year from the coffers.
The art work for the drawings was done on type of Velum with pen and ink. They didn't have Adobe Illustrator to work with. Relative to the price of labor and materials if it was done today it would be considered expensive.
At least Triumph put out addendum sheets for each of their parts books when they sent them to dealers. After 60 years of Triumph, I am still correcting my parts books, and finding parts with duplicate part numbers.