The general lines of diachronics of Western Lombard plural declension are drawn here, with reference to Milanese

Milanese

Milanese is the central variety of the Western Lombard language spoken in the city and province of Milan....

 orthography:

Feminine

The bulk of feminine words ends with the desinence -a; the feminine plural is adesinential. The last vowel finds its original length (in non-final syllable you can't ear the difference) that's often long when followed by a voiced consonant, short when followed by a voiceless consonant. When the stem ends with a difficult group of consonants you can see an addition of a final -i or of a schwa between consonants (for example: in Milanese

Milanese

Milanese is the central variety of the Western Lombard language spoken in the city and province of Milan....

sing. scendra, plur. scendr>scender). So in adjectives, plural form and masculine form are often the same.

Masculine

The bulk of masculine words end without desinences; plural masculine is adesinential. When the stem ends with a difficult group of consonants you can see, in singular and plural, an addition of a schwa between consonants. When the addition of schwa appears unnatural, they add a final -o (pron. /u/), that in the plural is -i.

The masculine words ending in -in, and some ending in -ett, have plural in itt. The masculine words ending in -ll have plural in -j (derived from addiction of -i and fall of -ll-; you can see the same phenomenon in the origin of determinate article: sing. ell>el, plur. elli>ej>i).

Some masculin words ending in -a can be unvarying (they often are words from ancient Greek or idiomatic words to define a person; e. g. pirla = a stupid).