According to RIC, this issue should have an N behind the bust.
Magnentius
A.D. 350
23mm 3.8gm
IM CAE MAGNENTIVS AVG, bare headed, draped & cuirassed bust right
VICTORIA AVG LIB ROMANOR, Emperor, bare-headed and in military dress, standing left with head right, holding standard with Chi-Rho on the banner in right hand and olive branch in left hand, to right a bound and bare-headed captive; N in left field.
In ex. RB
Cf RIC VIII Rome 193