From the start of 240 production to the end there were 4 different rear ends. 1030 SAE and 1030 metric. 1031 early and 1031 late. The 1030's have mostly interchangeable parts except for bolts. The 1031 is the heavier unit with wider gear teeth and larger bearings. The range of ratios runs from 3.31 to 4.10 with most getting the 3.54, 3.73,or 3.91 ratios. The info about the diff will be on a sticker on the back of the left axle tube covered by undercoat. The info on metric vs. SAE is discerned by looking at the bottom of the diff. If it is finned it is SAE, if not it is metric. The 1031 is not obvious by looking. There might be a 5th diff, I believe they made 1031 SAE for V6 models 76-80. Of course a V6 model is technically a 260 and not a 240. The late style 1031 started in 89 when Volvo quit using 1030 and went 100% 1031.