Having visited an abattoir (killed mostly cattle, but also sheep), and a bacon factory (killed only pigs), on a regular basis as a child, and seeing just how many men (there were no women employees back then) worked at the killing, I would not say it was a small number at all.
Many a butcher shop was staffed by one man, or maybe two or three if it was a larger establishment, so the number of slaughtermen was greater than the number of butchers.
Of course, these numbers would have changed over time, but back in the 1960s and earlier, there would have been sufficient numbers to definitely warrant a category.