Luce, S. B. “Heracles and the Old Man of the Sea.” American Journal of Archaeology 26, no. 2 (1922), 174-192.

Conclusion

I have sought to show that the vases showing Heracles in combat with “Triton” and the vases with the hero attacking “Nereus” really are portraying the same exploit, the two forms of adversary to Heracles being two distinct manifestations of Halios Geron [“Old Man of the Sea”]; “Nereus” being Halios Geron as he appears on the oenochoe of Cholchos, and “Triton” being Halios Geron as he is represented on the Olympia plaque. There is, therefore, no particular difficulty regarding this class of vases, when the true interpretation is reached.

Color version of a mid-6th century BCE hydria featured in the paper (fig. 5, p. 179). It shows Heracles wrestling Triton, while Nereus watches. Image found here.