Pilla lense numbers

I've recently come to own an older set (2 in fact) of Pilla shooting glasses with interchangeable lenses. Each set of lenses has a number ranging from 25 through to 65. I've looked online to see what they mean, and what light conditions each are suitable for but can't find anything as most info relates to modern Pillas. Anyone have any helpful info?

The number represents the % of light they let in. Eg lower number will be darker.

Use darker on bright day with consistent background colour typically