Some contemporary counterfeits I've recovered show the single raised line as does yours.
It's common enough that I believe it's a result of something in the clandestine manufacture process.
I had 12 different ones (mostly GII 1/2d copies) analyzed with XRF spectroscopy, all were composed of lead or lead/tin alloys (aka pewter)
with minor amounts of other elements.(inc. Sb, antimony)
I was told that the more minor elements present, the older the object likely is, makes sense to me because refining advances have resulted in availability of purer metals.
I think most early coin copiers used whatever scraps on hand for the crucible.
Want more period fakes? I often find them in the water near small bridges, as tollbooths would often be located here, and tollkeepers knew coinage better than most, apparently tossing these fakes into the drink.
unfortunately, many will have been cancelled by holing or flattening.
occasionally a beauty will be found, I have 2 exceptional GIIs, date showing and well preserved by salt water.
(and dozens of lesser, though identifiable coins)
Lead, pewter, and high carat gold being the only metals the sea treats kindly, most pewter blisters, but hard black blister is preferable to the flaking soft tan consistency of dry land recoveries imo.
