Help Identifying Rolling Block Proof Mark

[YohJules]

Hi Everyone -

I'm a first-time poster, and I'm hoping you all can help me identify the proof mark on this rolling block of my dad's. He's no longer with us, but his notes list the proof mark as "IV," but I don't see that particular option listed on any of the proof mark sheets I've found online. When you enlarge the photo, it no longer really looks like an "I," but I'm unsure what the letter actually is. There are no other marks on the gun at all. Any info you might be able to share with me would be so helpful.

Thanks in advance!

Jules