A white High Gothic numeral centred on the squad specialty symbol indicated squad number.
The colour of the left knee guard indicated company number in accordance with the Codex -- i.e. White (1st Company), Yellow (2nd Company), Red (3rd Company), etc. A white High Gothic numeral centred on the left knee guard also indicated company number.
The Brazen Skulls' Chapter badge was a large bronze skull centred on a field of sable.
Adeptus Astartes: Successor Chapters (Limited Release Booklet), pg. 21
Codex: Space Marines (9th Edition), pg. 73
Gaunt's Ghosts - Sabbat Martyr (Novel) by Dan Abnett