Source: ujjvala on Discord
The Saṃhitā-s in their current form don’t have a different metre. They just have missing syllable count (due to classical sandhi rules) that make a huge portion of the verses “metre-less”.
Many vowel sandhies in Veda are optional. Other than that, indraḥ atra would become indro atra (where the o is a short syllable metrically), but in classical sandhi, it would become indrotra.