Francis,
Sealing the hole is good idea even if it doesn’t go completely thru the rib as it could be hidden and made to look a little nicer. I have seen were the weep holes in the bottom rib are sealed by pressing a piece of lead shot of the appropriate size into the hole then shaping the softer metal to match the rib contour and finishing with a little cold blue.

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Larry,
What are the actual Stock dimensions DAC, DAH, LOP, of these three Parker's (26"28"30") you referenced as having a mid-bead 18" from the muzzle?...

Best, CSL
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Chris they are all over the place but I will list them here for you. I think the distance between the beads is an important factor because if they are spaced to far apart it becomes difficult to focus on both beads at the same time.
30"=1 3/8"x 2 1/4"x 15"
28"=1 5/8"x 3 3/8"x 15"
26"=1 1/2"x 2 1/2"x 14 1/4"

Each of those guns lettered with two beads and were spaced at 18". The next gun did not mention a second bead in the letter and may have been added later. Its spacing is 16" from the muzzle.
32"=1 1/4"x 1 3/4"x 14 3/4"

Optical illusion or aid for eye dominance??
 

Just my eyes, but both the PH 16- 26" barrels with two beads, and a 1950's M21 26" Skeet I bought recently seem to have a longer sight plane with the two beads. I looked at a 1950's M21 but 12 ga. today, 26" barrels but just a front bead- full length stock too- and it seemed "stubby"-

Wonder if the two beads help those with eye dominance issues?

It seems to me that the figure 8 picture and POI would be a product of the height of the front bead and mid bead, along with the angle of the rib vs barrel centerline.

Mid Bead
 

It always seemed to me that the Parker/King like way to install beads would be to have a jig that dropped over the rib and located the holes. I have an SC and a 32 inch PH that have beads separated by 18 inches precisely. Two obvious competition guns measure 16 1/2 and 14 1/2. Field guns are in the same range.
I think that the proper spacing for a mid bead is at the distance that it appears to be the same size as the front bead. This allows the "figure 8 " line up to work relative to the mount and also helps the dominant eye problem as Francis suggested.

Hope to see many of you at Baltimore

Best, Austin