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get-ipsw device-name generation-string firmware-version
get-ipsw iPod 2,1 4.0
Different generation strings:
iPhone 3G: iPhone 1,2
iPhone 3GS: iPhone 2,1
iPod touch 2G: iPod 2,1
iPod touch 3G: iPod 3,1
This can be used with idevicerestore (I haven't tried it though).