Fifty-five?

[Kish]

It's been mentioned nowhere else.

Oh, really?

Considering that Rich did have Roy mention Fyron's son over three hundred strips after the one you linked, after Start of Darkness had come out and any retcons he made would have been made, I doubt it's a mistake*, though the fact that said son has never appeared onstage is certainly odd.

*At least, a mistake made by Rich. It may have been a mistake made by Roy or by Eugene.