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M. Servilius C.f., 100 BC. Roman Republic, AR Denarius
Rome Mint

The control marks on this issue start with the Latin letters 
from the beginning on the reverse, paired with the Greek 
letters from the end on the obverse.

This goes in order A - Omega; B - Psi; C - Khi; D - Phi; etc. Crawford also notes that the letter D appears reversed.
On this example the D is NOT reversed.

Obverse: Helmeted head of Roma right, behind, control-mark (Phi); border of
dots.

Reverse: Two soldiers fighting on foot, their horses in the background;
M.SERVEILI.C.F in exergue, below, control-mark D; border of dots.

4.05 g; 21.4 mm.
Sydenham 602; Crawford 327/1; RSC Servilia 13; Sear Mill 206.