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:
http://videokarma.org/attachment.php?attachmentid=181249&stc=1&d=1388631303
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.

I wish my college still taught spice...They have us use multisim, and while it makes a lot of things simpler it becomes a headache if one needs to design a circuit with a component that is not in their limited database.