http://www.miata.net/garage/vlsd.html cliff notes look at the rear where the axles go if you do not see a bar it is a torsion if you see a bar its an open dif

I sold my 1.6 diff to some old fart on CL, I kinda felt bad about it, but it's not like it's a hidden secret on the forums.

FYI-

Torsen = Torque sensing

:ne: torsion