beardydave, Sierra diffs are a complete zoo. Ratios, LSD or not, push in or bolt on, there were almost all permutations produced at one point or another. Admittedly, some are rare. You can tell 7" from 7.5" by the shape of the housing cast. Here's a page that will, hopefully, shed some light on them.
http://www.super7thheaven.co.uk/Sier...r_Differential