Although forewings' pattern ant coloration are similar in two species, there are several differences which allow to separate two species with confidence even if actual size of moth is unknown.

1. Color of basal area of forewings. In Allerastria albiciliatus the basal area of forewing is dark and contrasts strongly with color of thorax. In Forsebia cinis color of basal area and thorax is the same.

2. Shape of postmedial ant subterminal lines. In Forsebia cinis these lines are less wavy than in Allerastria albiciliatus.

3. Coloration of forewings' fringes. In Allerastria albiciliatus fringes are obviously checkered, dark areas alternate with light areas. In Forsebia cinis fringes are almost uniformly dark.

I added a link to your notes on both species pages to help others.
Looking forward to your work on Cissusa. We had a good flight of the new sp early this year.
Thanks

I did not have the ability to measure the size with enough accuracy to distinguish between the two species -- size ranges given on BG info pages as forewing length 9-11 mm in Allerastria and 14-16 mm in Forsebia. But thank you for the ID assistance and I'll note that the current ID is a BG first for CA and a species with very few other BG records.