These cages are based on an old French design from the '40s or early '50s.

The Retro is a regular one-handed cage; you insert the bottle just like in any other cage, but it holds the bottle very securely due to the spring-like shape. You can spread the cage for an oversized bottle, or squeeze the cage inward for a better fit with (for instance) a metal water bottle.

Retro cages are made by hand from a single length of 3mm stainless steel rod and then electro-polished. If you examine it closely from all angles you'll see that it is, indeed, hand made.