I'm almost certain that's not a repair. The screening is far too even and matches up perfectly.
Repair on the plate would be by stoning off the big spot, then redrawing the dots either by hand, or by laying down another piece of transfer. It's incredibly hard to do that and get it looking exactly right.
As an aside, the one from Deans has a transfer laydown problem on the frame at the upper left.
The transfers were made by printing with very thick tarlike ink onto basically tissue paper, then laying that on the stone in the right spot with some solvent. When it was good and stuck down water was used to remove the tissue. Some times the tissue would tear, and that is probably the cause.
Depending on how common the spot is, it could have been on the master stone, either early and later fixed by remaking the master, or more likely later when a bit of something got on the stone
Comparing the cards on Ebay, there are a couple 350's with decent scans
http://www.ebay.com/itm/T206-Owen-/3...QAAOSwDrlZfjUb
http://www.ebay.com/itm/1909-T206-Fr...EAAOSw9fNZgleV
Both are screened differently from all the 150's I looked at, it's more noticeable in the face.
A few things I haven't quite figured out on it.
The shape is interesting, being an irregular octagon. That's odd as I can't think of a way that shape was created. It's too uneven to be a nut or bolt head, and probably too uneven to be from a bit of scrap paper from an octagonal hole punch.
There's also a missing halftone dot just under the lower left of the bog dot. That could be missing, or not picked up because of whatever caused the spot, or just another bit that didn't transfer properly.
Steve B