Unknown micrometer

[Tonyg]

Not sure that this is the right place to post this, my apologies in advance if I got it wrong.
I have this micrometer which was part of my dad's tools when he died, it must be at least 80 years old and I have no clue what it would be used for.
It was manufactured by Ambrose Shardlow in Sheffield. I have downloaded one of their tool catalogues (company closed many years ago) and can not find it, also done Google searches and spoken to many machinists with no joy.
The company also manufactured aircraft engine parts during WW2.
Hoping that some of our erudite members will know what it is for.