[null and void]

The keys St Peter is holding in the "And Finally" section.

They're for warded locks the silver one to fit a ward in the form of a cross, the symbology there is obvious, but the other dark base metal one has a 555 form, or perhaps SSS. What is the significance of that particular design?

IIRC, and not being a follower of numerology, I probably don't recall correctly, 777 is Divine Perfection - the province of Heaven, 666 total evil - the province of Hell, but 555? isn't that the number of mankind, or perhaps the province of Earth?

If so then that would imply that St. Peter in the artist's view has the keys to Heaven and Earth, both properly the dominion of God, but not to Hell, the designated dominion of Lucifer.

In numerical sequence Hell stands between Man and Heaven.

Odd and realistic, that...

[Hebrews 11:6]

Someone else may know and can provide your answers, but color me "ignorant." I've not inquired into numerology, because I am saved by grace through faith, and numerology is irrelevant to the only thing that really and ultimately matters to me.

I'm absolutely not being dismissive of your interest; rather, I'm simply explaining my knowledge-void. But since you directed it to me, I owed you a coherent reply.