Thanks for the pointer. I originally had “in 2001” for HP, so there was no distance measure. However, I’m referring to the earliest recordings I could track that actually mentioned Orpheus taming Cerberus with his lyra.
The two I found are:
The Georgics, a poem by Virgil, likely published in 29 BC and Horace, who lived from 65–8 BC.
I wasn’t sure about anything published before and since it’s all mythology, I went with the best historically accurate guess I could find and settled.