India Four Two: are you sure the lower setting was 997 mb.
I did wonder about that but here's what I saw:
After allowing for the fact that I misread the sub scale (!), 992.6 is still not right.
I went back to the hangar today and paid more attention during winding back. I was seeing about 30' per mb - HPa - sorry chevvron! - which is what I expected and then I noticed this at 1800':
The sub scale ends at 945 HPa, but you can still wind back onto the sub scale for a second time around!