This quarter-follis is the smallest coin issued by Constantine. RIC list the reverse legend for Trier only as PRINCIP, while this example is PRINCIPIS. Zschucke, in his book on Trier coinage
Die Bronze-Teilstück-Prägungen der römischen Münzstätte Trier has both legends listed as possible (this coin is #12.13) What is unusual is that I bought a lot of coins with this coin in it, but the coin was not even visible (it is partially visible to the left of the posthumous coin, but not easily identifiable) in the picture and I was pretty surprised to find it. The second picture below is the original auction picture.
I have a page about the circumstances of the issue of this coin.
http://www.constantinethegreatcoins.com/pacis/Constantine I
A.D. 313- 315
quarter follis
14mm 1.1gm
CONSTANTINVS P F AVG; bare head
SAPIENT PRINCIPIS; Altar with owl, spear across altar, shield to left and helmet to right.
In exergue PTR
Cf. RIC VII Trier 64