Roman Republic

Lucius Aurelius Cotta, 105 BCE

Fourree serrate denarius; 20 mm, 3.8 gmObv: draped bust of Vulcan right, wearing laureate pileus; tongs and star behind; all within wreath and dotted border
Rev: eagle standing on thunderbolt, head left; L·COT below, V to right; all within laurel wreath and dotted border
Ref: c.f. Crawford 314/1c; Sydenham 577a; Aurelia 21b
formerly slabbed, NGC ChVF, 5/5 strike, 3/5 surface

(click image to enlarge).  Even though the coin is clearly silver plated, with bronze core showing in spots on the obverse and reverse, at some point in the recent past an owner of this coin filed one of the serrations.

 

I might've done the same.  It doesn't harm the overall look of the coin since there are already irregular serrations, and getting a peek at the cross-section is pretty cool!

 

As for why serrates were made, it is unclear.  A popular opinion is that it was done to deter counterfeiting.  Where there's a will there's a way, or rather "where there's money to be made, fraud will hapen"... the counterfeiters prevailed.