And right of the bat, I spotted something interesting. In the second paragraph of the foreword, it states that shaving brushes is “…also required by the Defence Services in sufficient quantity.” Which I take to mean that at least as late as 1986, the Indian Armed Forces were issuing shaving brushes to the rank and file.
The Armed Forces aspect also shows in the two grades of brushes defined by the standard. Grade 2 was for normal marketing, while Grade 1 was for defence use. Which makes me – as a military man – think of the old trueism: MilSpec means the lowest quality the Armed Forces is willing to pay for – it does not mean ‘good’.
With the preliminaries out of the way, IS 4580 goes on to describe acceptable grades of nylon monofilament and bristles (as defined in IS 1844). It also points out that the handle can be made from high density plastic or timber – but if the manufacturer choose wood, only a few woods were acceptable.
The standard also goes on defining the material for the ferrule, the cements used to set the knot, and other details. It also gives the tolerance allowed on knot size – a knot between 15 and 40 mm, for example, could vary in size with as much as 2 mm.
IS 4580 also goes into details on how to test shaving brushes. Tests includes pulling on the knot, baking it to make sure there is no creep in the setting compound, and checking for dyed bristles.
Oh, and when you package your brushes you should dust them with DDT-powder. Because DDT kills bugs as well as people, I guess.
You can read all 22 pages of IS 4580 at Archive.org.