It's Solus Prime's insignia.
Which raises options:
1) He's "The Forge of Solus Prime" her famous ancient artifact Hammer.
2) He's a different hammer she created. This would still be very impressive as she's a demigod of weaponsmithing.
3) His robot form worships Solus Prime and it's a religious affiliation mark.
-ZacWilliam, all of those are fun.