After more research, it looks like the bottom ring either wasn't there or was painted copper for the hero prop/special effects back in 2002.
Shots of Sam Raimi rolling around the hero prop
Shots of what seem to be the stunt prop (you can see the bottom ring is white in this one shot, perhaps not yet painted or perhaps worn down from use/rehearsal— although I'm skeptical of this since the top rings are ALSO painted copper, so not incredibly reliable for the hero prop.
And these seem to be the same hollowed out prop as this:
And again the bottom is painted copper, but then again so is the top.
So the stunt props had the ring on the bottom, but the real question is if the hero prop does— the stunt props could have the ring, and they painted it copper because it's not supposed to actually be there.
I'm gonna assume the bottom has the ring unless proven otherwise.
Also, the more I look at that bomb from the Heritage auctions site... the more I think it's actually from Spider-Man 3. The screws don't match 2002, the rings have a height and taper not seen on the original prop, and the green ribbing between the cracks is a detail from 3, but missing from 1.