I don't think serial numbers would help in this case, as the converters are stolen, precious metals extracted, and then those metals are sold. The bit that would likely be stamped with a serial number is likely discarded.
Once you have a small sheet you go to a scrap yard and say your dead grandpa was a hobby jeweler and nobody wants to continue the hobby. The important part is make it into a form where nobody can recognize it.
The grade isn't as pure as industrial processes, but it isn't too bad. Likewise, while the yield isn't good, even if you collect 50% the value is high enough.
