Well, it's taken me 60 odd years to hear about this instrument, which is used on the Yes albums Tormato (1978, deep and compressed in the mix), the live Yesshows (1980), and Wakeman's solo album Criminal Record (1977). It also appears on three albums by the band Earthstar: French Skyline (1979), Atomkraft? Nein, Danke! (1981), and Humans Only (1982).

The Birotron also appears on two Tangerine Dream albums: Force Majeure (1979, featuring the male choir and violin sounds throughout the album), and Hyperborea (1983) showcasing the male choir sound again.

Despite deteriorated sound due to tape damage, it has made two more known of recent appearances, in 1990 and 1999.

There's a good wiki page on it and here's a couple of other great articles I found:

https://www.believermag.com/issues/200706/?read=article_collins
and
http://articles.courant.com/2010-01-18/features/hc-birotron2.artjan18_1_biro-dicey-proposition-rock-star

To hear an example, go to http://www.mikedickson.org.uk/tron/
then click on the Vices link, then Birotron Choir.

Mike Dickson of Streetly Electronics kindly made this for youtubr:
https://www.youtube.com/watch?v=0SqLl5fOokA

Very interesting. Only two functioning units left in the world and the tapes are worn out.

Pardon my ignorance but what is the difference between a birotron and a mellotron. Just different makes or brands?

For a more detailed explanation, please read the article at believermag.com that I already gave a link to.

The Mellotron uses pieces of tape to record samples of sounds that only last for 8 seconds. The Birotron uses recordings on 8 track tape cartridges, so it holds the sound indefinitely when a key is pressed, allowing an endless loop for notes with infinite sustain. Also, the keyboard can be played quicker and therefore faster than that of the Mellotron.
Both instruments use samples of real instruments/voices to reproduce sounds.

The Mellotron was initially a rip-off of the Chamberlin.
In 1962, Bill Franson, Harry Chamberlin's salesman absconded to England with 2 Chamberlins, relabelling them 'Franson' and getting an engineering company, Bradmatic Ltd to manufacture the playback heads. Bradmatic Ltd eventually became Streetly Electronics.

It's a long story. Read it all here:
https://en.wikipedia.org/wiki/Chamberlin

Cheers Steve.