Chris Henshall (1972) remembers...
I was admitted as a Choral Exhibitioner in 1972 – as a baritone but then “converted” to Alto either at some point in my first year, or at the start of my second – mainly to help fill the gap left by Robin Page who had been admitted at the same time as me in the first batch of Alto Choral Exhibitioners but then had laryngitis and never managed more than a few croaks in his first year. In our second year (73-74) it became clear that – sadly – his Alto voice was not going to recover, so he gave up his exhibition and a special competition was held at the start of the first term to see if there were any replacements already up. That resulted in Jeremy Points  being awarded a Choral Scholarship as an Alto and joining the choir during the first term of 73-74. Jeremy and I tended to take the higher lines and both of us could manage high Gs without too much pain (for us – I can’t speak for the audience), and Peter wrote a number of pieces for the choir in the following two years that exploited this. There were also some memorable arrangements of the classics – sometimes very little transposed (and in one case, I remember, with sections transposed to different degrees and a few bars of PAT chromatic harmony added to “smooth over” the transition!). These arrangements and our performances of them raised a lot of eyebrows – it was great fun!
My understanding was that, while Alto choral scholarships were not awarded before 1972, the choir did nonetheless sometimes perform with alto parts taken by other parts singing falsetto. I remember Peter saying that one or two were particularly good at this, though I can’t remember who they were and I don’t think any were still up when I joined. So Peter must presumably have had some music with Alto parts for the choir to sing in this configuration – though probably not a lot. As others have suggested, it was probably by way of trial runs for the full “conversion” of the choir to AATTBrBrBB – which of course involved an enormous amount of work for Peter to arrange or re-arrange the entire corpus of music.
In The Caian of November 1974 (covering the period October 1973 – September 1974), Peter Tranchell wrote at the beginning of his report on the Choir and Music in Chapel:
This was the second year in which altos formed a regular ingredient of the choir. On occasion the altos were supplemented by the 4th-year ex-Organ-Scholar [Norman Harper 1970] and by the Chaplain [Revd Brian Watchorn]; and baritones by an ex-Choral-Scholar Research-Student [Nigel Burton 1965].
The absence of real tenor voices (a function, it seems, of the incompatibility of the tenor voice with good A-level results!) involved a considerable re-organisation of the repertoire, but sometimes could be compensated for by the division of the altos. Still, in general, the altos were unanimously loth to sing the lower parts, since this tended to affect the overall control of their voices; and they felt more comfortable in their own natural range, which during this year were nearer to that of a boy’s treble voice than to that of a counter-tenor. It was rather remarkable to hear full-blooded high E.s, F.s, G.s and G sharps emanating from them when necessary.