The coin you don’t think is a cud really really really looks like a cud…. There’s no extra metal involved, the cud is caused by part of the die breaking away after a die crack forms. It has to be at the rim and will create a raised area that the metal has flowed into. The pics of that nickel aren’t the greatest, but I’d put my money on a cud for that coin. As for your coin, yeah it does look like a die crack, but the only way to take a flaw on the holder away is to take it out of that holder. It could be that the lack of any other findings for it comes down to it being a proof. For proof coins perfection is the goal. A flaw like a die crack might appeal to a small segment of collectors but might turn off most. Just a thought there’s…
The 1971-S has a cud. The 1973-S may have a crack or we are seeing junk on the coin. The images are too poor to really know.
How about posting the link to both of the Error Jeffs. That way we can look at the reverse. Both of the errors are identified by the reverse of the coin. As far as the die crack. That one is pretty large, looks like it extends from the field and into the hair. See if you can just focus on the coin. Might have to shoot it at a different angle, and slightly different light.