Further to my previous posting, I am now 110% sure that they are 5 BA.
I have a beautiful boxed set of BA taps and dies that is donkeys years old and seldom used. The series runs from 0 BA to 22 BA, but my set only goes down to 12 BA. The smallest I have ever used is 8 BA, and that was in my teens when I was building model boats and aircraft.
Not many people realise that the BA series is a METRIC system, and I never realised until I noticed that 0 BA has 25.4 threads to the inch, and of course there are 25.4 mm to the inch.
Incidentally 22BA has 254 TPI (!!), WOW! Totally irrelevant to Scotts, but interesting, useless information that my brain seems to be full of…
When the UK “went Metric” the BA system was declared obsolete! It should have been embraced as an existing Metric system, in regular use since 1903.