It's a 'Computer'
Hi HeliStudent,
It's a circular slide ruler type of computer that surround the collective pitch indicator. With noting the outside air temperature and pressure altitude, the pilot can determine the maximum gross weight at which the outside condition will permit. you can also get from it the maximum pitch setting, actual gross weight (under hovering condition) and density altitude.
I'll try to get a hold on my 35 years old training manual and get you more details.
Very clever at the time.
Cheers
kami