Big one. These were overstruck onto the flans of coins of pretenders and usurpers to allow the money to still circulate. Another way coins were recirculated was to cut out the name of the usurper. In East Anglia lots of tiny 10mm coins cut out of Magnentius Folles are found which is probably for this reason.
The previous coin is not visible underneath. I was hoping I had a Carausius II but, alas, nope. They've had a good whack at the correct legend (DN CONST/ FEL TEMP) but havent done so well (DN CIH(retrograde C)HM / I(F/T cross)(retrograde E)EL TMP). Mint mark TR... probably imitating Trier. Very well rendered fallen horseman though - one of my nicest, embarrassingly.
Heres pictures. I finally set up a board with a background so I can standardise my pictures.



How did I get it? Well, remember I bought those smashed Denarii? They took a couple of weeks to arrive and so as a sorry the seller sent me this. Its so much nicer than the original purchase!
(I did put a bid in for some Magnentius follis cuttings but its gone to high. I've also bid on something much...rare, which I hope to win)