Playing non-parallel harmonies on a harmonica

In this recording of “It’s only a Paper Moon” by Max Geldray from a Goon Show in 1957, he plays a very nice harmony line along with the melody.

This is unique among all the hundreds of songs Geldray played in nine years on the Goon Show. Every other song featured only a single voice, no interweaving melody/harmony lines like we hear here. And although I haven’t exactly made a study of other harmonica players, I’ve never heard anyone else play anything similar.

Any harp players here who can explain how he did it? Is it an unusual virtuoso technique? Is it something special about that particular song and its harmony line?

Geldray played a chromatic harmonica, but I don’t think that’s exactly an explanation for this, and it also doesn’t explain why, if he could do it for this song, he never did it for any others.

Thanks.

No harmonica experts? Have I stumped the Dope?