Except those objects were found in coin hordes and the graves of rich aristocrats, and must have been too valuable to be a simple knitting tool.
And for some reason, this style of knitting would have then disappeared until it was reinvented the 16th century.
If it’s truly such a mystery, is it at all possible these only exist because they looked interesting? Just a knick-knack for your shelf?
It's one of the most convincing theories, but also a bit unsatisfying. The question then becomes, why they were made in relatiely large numbers (so that hundreds could be found) with that very specific shape in different parts of the empire.