Gearbox tech question
 

A few years back when I bought my racecar, it came with a spare gearbox.
Its been sitting in my basement since then, untill the other day when I ripped it apart to determine what the hell it is.

SA style shifter model
Gear teeth:
28/13
26/20
21/25
17/29
31/15

At first I was like "Cool, another racebox" based on the 28/13 gears which in my simple mind gives a ratio of 2.153:1 Pretty long even for a racebox. But somehow, based on the other gears, I think I've got something wrong, did I miss an extra conversion ratio or what ?

What do I have here ?