I've been looking for rack rails that allow half RU spacing for some time. Just got in a pair of the AudioRax rails (
https://www.audiorax.com) and they are just what I've been looking for.
For ventilation purposes, they allow me to place standard rack gear in a rack leaving one half RU between each unit (instead of one full RU, as would be the case with "normal" rack rails). For example, I have three tube units racked as follows: half RU space, 3 RU unit, half RU space, 1 RU unit, half RU space, 2 RU unit, half RU space.
Packaging was excellent and shipping was fast (3 days to California).
If you are trying to answer that age-old question, "how do I leave 1/2RU instead of 1U"- this is the answer.
I have no association with AudioRak; just a satisfied customer.