I believe an orchidometer is used to try and assess whether the size of the testicles are consistent with the age of the male.
The doctor would choose a bead size which matches the testicle size, by laying the beads against the testicles and finding the closest size.
As you guys know, testicles sizes vary quit a bit.
The orchidometer can be used from an early age to adult.
Doctors can generally have a good idea by feeling the testicles, but if youngsters are examined each year the orchidometer reading is more quantitative.
An orchidometer was only used once when I was examined when young.