Sadly we will likely never know the details. Could be a former master brought out of retirement or who was now a supervisor but sometimes had to work, or an engraver that normally worked on the precious metals, or a gem engraver brought in to assist, etc, etc. Bastien simply called such people engraver A and engraver B, etc and tracked their careers at the mint. SC
Here is another common and inexpensive Constantinian coin from my collection. The coin shows a common standard bust, which is, however, exceptionally nicely engraved and the relief of this coins is unusually high. Licinius II (Alexandria):