I looked everywhere here before and could not find any info on telling the two differentials apart empirically. I finally found a great writeup on another forum, felt like re-posting it since it is such good info. This is from RX7 & 280ZX junkie at torquecentral.com. There were pics there as well, but I just copied the text below in case the link goes bad..

http://www.torquecentral.com/showthr...2285&highlight

"I have 2 diff's on my table, and I measured all the features.
This is what I found. 86 n/a as compaired to a 87 T2.
The drive shaft flange.
N/a is 1160MM diameter
T2 is 1240MM diameter
The stub axle.
Resess where grease cup goes is n/a 850MM
T2 is 885MM. see pics.
Studs measured outside to outside is.
N/A 880MM
T2 905MM
Markings were different. see pics
Phisical appearance is different, one has more ribs,
different gussets.
Hope this helps to identify the differences.
Lloyd"