I think it is a random die break in this case, attached to the break at the center of the X of
IMP XI, rather than a guideline, since guidelines are usually longer and circular, maintaining a constant distance from the center of the die.
The line from the chin of the
snake to Salus' knee on the
rev. also appears to be a die break.
Fortunately for me the dealer
had not noticed the
legend error, with the result that there was minimal competition for the coin!