1843 Mexico City 8 Reales Counterfeit?

Bob, I'll concede that you shouldn't expect long term durability out of a $10 digital scale, and I've also experienced the sudden onset of innaccuaracy (or complete failure) you describe. But during its useful lifetime of such (and some styles seem much better in this regard than others) I've had excellent results in checking them periodically with standard weights as verification of their continuing utility. I'd further remark that although most have a calibration mode, that's a superfluous feature if they're simply becoming erratic.

What interests me is that all the coin shops I frequent (and these are really reputable places) have on the counter digital scales that are hardly better than what we're discussing, and they get years of constant use from them, through multiple validations by the state authorities. No shop I patronize seems to employ an Ohaus, unless it's in the back room.

You're "preaching to the choir" on that point with me. Not to belabor the point, but I'd add that maybe every dealer ought to have the capability for the protection of their clients. But obviously that's not going to happen across the counter, where dealers have to make snap buying decisions regarding material that walks in.

The pictures are much better and the coin looks a lot more believable now.

I do note the presence of numerous small raised dots in the fields of the reverse die. These could be rust damage top the die. I see none that point to another method of formation.

The area at the top of the assayer initial D - is very odd looking and I am unable at present to come up with a satisfactory theory about how a die crack could produce what I see here. It appears to be some sort of repair to the die after a small part of the die chipped away. Have you examined the area using a binocular microscope? It appears to be a feature that is raised above the level of the field on the coin. In that case, it would be a depression in the actual die. The thing I can not resolve is the way it appears to be rounded. It looks like a tiny weld used to repair the die - but welding like that was not possible in the first half of the 19th century. Just makes me wonder.

I have studied far more Mexican edges than those from Chile.

This edge detail makes me rather concerned. The circles are in many cases not actually circles. They look like they are squared on the outside with a circle drilled into them. That edge on a coin from Mexico, Bolivia or Peru would move it into a "counterfeit" category.

Were you able to locate two overlaps? Are they of equal length and opposite one another?

Interesting overlap - I was able to align your pictures at the same scale so that the edge overlap can be seen. It appears to be the same length and the overlap priority is correct as well. This points to the use of a two die edger (casting machine or edge mill). However, the shape of the circles remains a problem in my opinion. I would suggest that other collectors with the same date coins need to post similar pictures to see if the genuine edge was created from a matrix block or not.