Is this tool for tinsmithing?
No. It is not listed in any Manuals for the tinsmith trade or catalogs of tinner’s tools. I have seen countless references to it being for seaming but for that purpose there are seaming pliers. Some say it is to hold to pieces of tin when soldering. Any tool similar to pliers could be used but why would so many tinsmiths have this tool available for their use. Some people have said they are an unfinished bullet mold. That has evolved because many are marked Dixie, and they associate that with Dixie Gun Works that sells muzzleloading supplies. The Dixie was a tool company just as P. Lowentraut was a tool company. Here is a page from the Lowentraut catalog for 1891 with the pinching irons illustrated.
How are pinching irons to be used? Here is an excerpt from Ward and Locks Home Book on hair dressing from 1882.
So, you may use them for tinsmithing but their original purpose was to straighten hair.