I don't recognize the guitar Yui has (it looks like kind of an odd fantasy cross between a Gibson Firebird and a Gretsch Duo Jet to me), but Mio's got a classic left-handed Höfner "violin" bass, of the sort made famous by Paul McCartney back in the Beatles' heyday. Always an "oh hey!" moment to see the K-On! kids specifically rendered with instruments other than their usual ones.
