I haven't looked into Cyclommatus that much.
But i have seen a seen a study on Cyclommatus metallifer before: Taxonomic study on the subspecies of Cyclommatus metallifer (Boisduval, 1835) from Indonesia (Coleoptera: Lucanidae)
maybe you can start there.
There should also be pictures on Japanese Auction.
Btw the way this post was for Lucanus lol.