The discovery of clay moulds at La Coulonche (Orne) in the mid-19th century attested a workshop producing under the first Tetrarchy cast nummi of Diocletian, Maximian, Galerius and Constantius. It is unique for continental Europe, but because the material was dispersed after being recorded, and a significant part of it lost, the find remains little known to scholars. The interdisciplinary research presented here, based on 147 moulds and fragments preserved in four collections, examines the way in which the moulds were made, the origin of their clays, and the nature of the alloy used for the cast nummi. The coin issues used to make the impressions on the clay indicate two phases of production (c.298-300, c.302-303), as is confirmed by study of contemporary hoards of nummi. The use of the casting technique and of an alloy with very low silver content show that this was a forgery operation, the production of cast nummi being explicitly prohibited at least as early as Constantine the Great. Worsening price inflation at the turn of the 4th century would have encouraged counterfeiting operations in the provinces, but the limited evidence for the phenomenon, confined as it is to the north-west of the Empire at the time of the first Tetrarchy, suggests more local causes. The only other forgery workshop known in the west was at Duston (Northamptonshire), which ceased production of cast tetrarchic nummi at the same time as the workshop at La Coulonche, in 303, a year which also marks a Schatzfundhorizont in north-east Gaul. An inadequate supply of official nummi to the north-west provinces until about 303, aggravated by the delayed spread of nummi to Britain (and to certain coastal areas of the continent) held by the rebels Carausius and Allectus until two years after Diocletian’s monetary reform in 294, could explain these isolated operations on either side of the Channel. Finally, the increased output of nummi at the mint of Trier from 303 ensured a better supply of billon laureates to the north-west provinces, as the hoards show. The reconquest of the north-west and its economic reintegration into the Empire will have caused the ephemeral workshops at La Coulonche and Duston to close.