sierra LSD diff- help!

[onedayTM]

I have just bought a diff off ebay advertised as sierra cosworth / granada with
serial nos ending in AA. I have opened box today and I cannot see any tag or anything else with such nos. I have since checked online and it seems that this code is for a 3.92 LSD 7", but mine was stated as being 3.62 and also it seems that cosworths/granadas were 7.5" and this one is a7".
Is there anyway to tell if this is a LSD other than serial nos or have i beed duped?
cheers
tony