Did some current and voltage drop measurements on a selenium rectifier that I had laying around, and did some curve fitting to create a 3f4 Spice model. the diode was marked "RCA 752652" and has 6 fins.
* seleniumdiode CircuitMaker model
.SUBCKT Xrca752652 P K
Bp P K I=(12.42276354/1.0e4)*uramp((V(P,K)*0.55)+(-2.938702085))^2.4
.ends Xrca752652
The curve as I saw and plotted it:
Not that you're going to use selenium rectifiers in a new design, but so you can compare a modern silicon diode circuit with an existing selenium diode circuit.