Earlier quoted context omitted.
One thing to keep in mind is the geographic distribution of these things. They're mostly found in the province of Gallia and Britain, and not in other regions. There'd be a lot of metalworkers in Rome and the surrounding cities in central and south Italy, but no artifacts like this have been found, and they've found > 100 of them now so the distribution is probably significant.
The cities likely had some form of guild system to prove claims like this. The middle of nowhere you could prove yourself with a standardized piece of work. ___ Though personally I like the "glove grandma" video of her crocheting gloves with the pieces.
Why none in Roman North Africa or all the other provinces in the east which were also "the middle of nowhere" with the same needs of metal work as Gallia/Britain.