Their closest relative seems to be C. Septentrionalis, the Blue Line Angel which is pretty common in Japan, China and Taiwan although its entire range is unknown and the Vietnamese specimens look a bit different. They’re one of my favorite angels and one or more members of the “Velvet” Angels frequently hybridize with the Blue Line, the hybrids of which are sometimes known as “C. Cephalareticulatus” and “C. Chrysocephalus.” Hybrid phenotypes range from a more exaggerated and wildly patterned Blue Line-leaning, to an almost entirely Black / Grey Velvet appearance and many in-between. The taxonomy of this group is murky to say the least.
The hybrids are called “Maze Angels” and the ones with more flashy patterns are pretty amazing.