The song Inkomo Zodwa was recorded by Miriam Makeba and the Skylarks in March 1959 and features Spokes Mashiyane on the pennywhistle. It is accredited to the South African playwright Gibson Kente. I originally got hold of this track on The Rough Guide to the Music of South Africa, and you can too (although I think it was an earlier version and the tracks have changed now).
Spoke’s whistle part is a constant solo throughout the song, playing in the lower registers during the singing, and rising up in volume and pitch in between. The key signature is somewhere between F and F#, and if we assume F, then Spoke’s whistle playing goes right down to low F. We’re talking Low Whistle territory here, and I must say that I’m rather surprised by the idea that Spoke’s had a low F – I mean, these aren’t so easy to come by nowadays and I daresay that Overton didn’t exist back then (if you have any ideas how this was played, I’d be very happy to hear them!)…
Leaving the academics behind; I’ve party transcribed, partly made up (the low bits of) the whistle part so that this can be played, along with the recording, on a Bb whistle – it’s sure to bring a tear to your eye. You might like to fractionally pitch shift it to get it in tune. -130% semitone did it for me using the demo version of Ableton. [I've subsequently learnt that this kind of manipulation is possible using Audacity, which is free]
I hope you enjoy trying to play this part and that it encourages you to listen to some of these old kwela recordings.