I've never heard of different ratios on Discovery diffs, I suspect there is a miss print somewhere.
Until the independent suspension vehicles came out there were basically 2 diff ratios on Land Rovers (with the exception of very early 80" and the 101)
4.7:1 fitted to leaf sprung vehicles and 3.54:1 fitted to coil spring vehicles. The exception was the Stage 1 V8 that had leaf springs and a 3.54 diff.
There were some other ratios that were fitted to Rover saloons that will fit.
Wammers, what makes you think your 200 has 3.45 axles?
I would expect an NRC2750 to be a new diff, an NRC2750R to be a recon, that is usually the meaning of an R on the end of an LR part no.