Cool watch!! I believe the altimeter function is sort of automatic. There is a tiny air pressure capsule inside the watch that engages the red hand.
The outer bezel (black) indicates the amount of feet x 100. So if the red hand points at 20 you should be at a 2000 ft altitude.
Looks like you're at an altitude of between 200 and 300 feet now, standard sea level. Does that seem right to you?
The barometer I'm not sure. Maybe someone on WUS has the original manual.
Since it has no other crown,the seconds hand alike pointer works like as Vintage1982 described, I would second that.
See how the bezel at 760mmHG =1ATM sea level is at 12 o'clock mark. So you are at 300feet above sea level as the watch indicates.
The 100 mark on bezel is actually on the 59 min marker.
0/60 min marker is 1 and at 760mmHG-sea level.
So that center hand moves according to the pressure change and corresponds also to altitude on the bezel as altimeter. For example the least pressure on scale there is 560mmHG which corresponds to appr. 8,400 feet.
And at the other side it may also show depth according to increasing pressure under water : 790mmHG corresponds to depth 1000 feet,which might be also the limit of the watch as pressure and water-resistant.
The table below indicates air pressure at elevations below and above sea level.