If the entire axle has been swapped it will immediately be apparent - the Series 2/2a/3 is about two inches wider track, and the wheels will protrude slightly beyond the mudguards, depending on tyre size. The Series 1 rear axle is semi-floating (except very late station wagons) whereas Series 2 on are fully floating, meaning that the hub on Series 1 is flat, where as on later ones the drive half axle protrudes through a bolted on drive flange and has a large nut covered by a pressed hubcap, the same as the front.
If by 'diff' you mean just the differential centre unit that bolts in to the axle housing, while there were minor changes through production, these are a bolt-in swap all the way to the end of Series 3 production, except for the V-8, which has a different reduction ratio. Note that non-standard diffs may have been fitted at some time in the past, particularly to suit it better to the Holden engine.
Most likely the brake assemblies have been swapped from a long wheel base from Series 1 onwards, which had 11" brakes - wider 11" brakes came with the 6 cylinder engine during Series 2a, and were fitted to late Series 3 lwb with four cylinder engines. Fitting lwb front brakes to swb Landrovers is a simple and effective way of improving brakes, but needs the master cylinder swapped as well.
Hope this helps.
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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