There is an interesting series of SOL INVICTO coins without a mintmark in the exergue. RIC VI attributed these to the Trier mint; but they were also struck at London, Ticinum and Aquileia. These coins were originally attributed by style to the Trier mint by Voetter. Bastien wrote two articles in the 1970's that showed examples (based on style and die linkage) from Ticinum and Aquileia1&2. Most recently examples from London have been noted3.
These coins were probably struck around A.D. 311- 312, about the time that Constantine opened the mints of Ticinum and Aquileia. Gautier believes that these coins from all the mints were struck simultaneously, so 311- 312 at the earliest based on when the mints were taken over and the sequence of field marks from London issues. Bastien even stated that these unsigned coins were struck for significant occasions, like donative. There was a donativum in late A.D. 3124. There is a possibility that these coins were also minted at Lyons and perhaps even Rome.
These coins were initially struck at 1/96lb and then reduced to 1/72lb. The last picture below is from an auction that has the two different modules side by side for easy size comparison.
here is the description for the first coin below--
Constantine I
A.D. 310-13
22mm 4.7gm
CONSTANTINVS P F AVG, laureate and cuirassed bust right
SOLI INVICTO COMITI, bust of Sol r., radiate and draped, seen from rear.
RIC VI Trier 893
1 Pierre Bastien, "Une emission de folles sans marque a Ticinum en 312" Schweizer Münzblätter 20 (1970)
2 Bastien, "Folles sans marque emis par Constantin en Italie" Schweizer Münzblätter 24 (1974)
3 Georges Gautier, "An Unpublished Nummus of Constantine I of the Mint of London" Numismatic Chronicle 152 (1992)
4 Bastien, “Interprétation des antoniniani sans marque émis à l'atelier de Lyon de 275 à 294†Festoen opgedragen aan A. N. Zadoks-Josephus Jitta bij haar zeventigste verjaardag Scripta archaeologica Groningana 6 (1976) pg. 75