Magnentius was elevated to Augustus in Gaul by his troops in 350AD and quickly captured a large chunk of the roman empire by killing Constans and then marching on Rome (while Constantius II was predisposed fighting in the East). This period in Roman history was usurper rich and Magnentius had two sub-usurpers to deal with - Nepotian, a Constantinian in Rome, and Vetranio, a loyalist to Constantius II. The Nepotian revolt was quickly squashed, but Magnentius was defeated in the field by the forces of Vetranio (by this time turned back over to Constantius while Vetranio went off to live a quiet life in retirement) and forced back to Gaul. He would spend another two years pottering around Gaul before finally being defeated once and for all at the Battle of Mons Seleucus in 353AD. He then promptly fell on his sword.
Following the revolt, much support was still around for Magnentius - he had been tolerant of paganism and was a half Brit half Gallic celt, so the people liked him. In order to crush this remaining sentiment, Constantius II sent out a fearsome civil servant called Paul the Chain, who brutally suppressed support for Magnentius and fervently carried out a damnatio memoriae
against him which resulted in the destruction of much of his coinage.
In Roman Britain, small change was in constant short supply. The London mint didn't put out enough coinage to meet demand and the flow of coinage across the channel was insufficient too. An enterprising tribe in East Anglia had a solution for this - instead of sending the Magnentian coinage for destruction, they would put a punch through the eye (symbolically destroying his image) and chop it into minims. For some reason, I have only ever identified Magnentian minims cut from Chi-Rho folles - perhaps the Chain was only content for these overtly Christian coins to remain, or perhaps it is simply easier to tell a chi-rho when you see one.
I have posted a couple of these rare minims before and they do show up on eBay once in a blue moon, but I hadn't ever seen one which is identifiable as Decentius (Magnentius' Caesar and a somewhat rare figure on coinage, even in Britain). These two coins come from a lot of 18 barbarous minims from eBay and one reads, very neatly, DEC[ENTIVS].The other is quite good too - it reads [MAG]NEN[TIVS]. Both have the Chi-Rho on the reverse, one with the base and the other with the bottom of the loop of the P, the top of the right Chi and the omega. Once you know they are out there, these fragmentary chi-rhos aren't so hard to spot.
I will be keeping my eyes peeled for a cut up Constantius II chi-rho follis from the revolt of Poemenius - these folles aren't so rare as they were not withdrawn after Magnentius was defeated, but there was also no reason to cut one up, so I'm not sure if I will ever find one. Luckily, the legends are different to Magnentius' on both sides, so any part of one will be totally identifiable.
Decentius, AD351-353, 0.95g, AE9, Cut follis
[DN] DEC[ENTIVS NOB CAES], bare head right
[SALVS DD NN AVG ET CAES], large Chi-Rho, [alpha] and omega across.
Magnentius, AD350-353, Trier Mint, 0.86g, AE12, Cut follis.
[DN MAG]NEN[TIVS PF AVG], bare head right
[SALVS DD NN AVG ET CAES], large Chi-Rho, [alpha] and [omega] across. TRS... in exergue.
Fun closing fact: Paul the Chain would later be burned alive by Julian the Apostate for his brutality.