Jestyn, the nurse who is at the very centre of this mystery. Her real name was Jessica Ellen Thomson (nee. Harkness). She was born in 1921 in Marrickville, NSW.
She signed her name "Jestyn" in Alf Boxall's copy of the Rubaiyat. The name Jestyn likely comes from a combination of Jess and Tina, a nickname she was given because she was short.
She trained as a nurse at the Royal North Shore Hospital in Sydney, although no record of her graduating can be found (apparently there is another lady with the same name that graduated the following year, but who is a completely different Jessica Ellen Harkness, although I am unable to independently verify this).
She left due to being pregnant with her first child, Robin. At the time she was unmarried although she was using Prosper's surname. This was due to the stigma attached at that time, to having a child out of wed-lock.
Jessica did marry Prosper in 1950, after his divorce from his then wife Queenie came through.
She passed away in 2007.