Berber languages share a feature with Semitic in that plural formations are a bit... weird. You have suffix formations, internal formation (by insertion of a before the last consonant) a combination of both (though arguably only in a few languages) and then there's the suffix formations that have emphatisation of a root consonant in the plural.
A good example of this emphatisation is the word for 'dog' in Central Atlas Berber sg. iydi pl. iyḍan. As you can see, this word has masculine plural suffix -n, vowel change of the final vowel i to a and also change of the consonant from d to ḍ.
The phoneme /ṃ/ is nonexistent in most Berber languages, but Zénaga has a [ṃ], people have wanted to analyse this consonant as subphonemic but have thus far been unsuccesful. What's more, is that some words actually show emphatisation in the plural with the consonant m.
As is found in the word for 'beard': sg. taʔmmärt pl. tuʔṃṃuṛaʔn 'beard'. Central Atlas Berber sg. tamart pl. timira. It seems clear that this is the same morphological process as the emphatisation that we find in other Berber languages. If this is true, than surely /ṃ/ should be reconstructed for the Proto-Berber languages as *ṃ.
Zénaga also has an emphatic [f̣] (that is a f with a dot below it, fonts aren't kind for such foreign characters). I'd have to search for plurals with a similar alternation, but I expect to find something similar as with the *ṃ.