Wow. Thanks for the link. This guys know how to handle a mellotron. I like the vintage melotron made in the 60´s. I once builded a little melotron on my one in my childhood. It depended on a tape recorder. I only recorded one sound(instrument) and played it back on an older keyboard witch I connected over different resistors to the motor of the tape recorder. So i was able to produce different sound tunes from the engine speed.
The sound I used in this song comes from a vst called “RealiWhistle”.
It´s a very usefull tool. BUT:

Be careful. It is not working with newer daws and kontakt versions.
It makes heavy midi errors like holding one tone.
So what I did:
I recorded every single tone with articulations and play it on my own sampler.
I will upload the samples if you are interested.
Just let me know.

Thank you, Mark.