Wow! You were all right weeks ago! My Grandfather was Harold Arthur Forrest Koselj. I had no way of identifying him as such a few weeks ago but I finally received the divorce file yesterday where Eleanor's first husband Frederick caught up with her. It mentions that she gave birth to two sons in her second marriage and there is a handwritten note stating their names. So far, this is the only information I have, I don't believe that Elsie the mother was ever a Forrest. The Father on the certificate is Ivan Koselj. Could the mother have simply lied about having previously been a Forrest, in order to make passing him over to the Forrests a bit easier? Irene/Eleanor has form for making up maiden names, the name Hardinge that she was using appears never to have been hers to use and was borrowed to enable a second marriage to Harold Forrest Senior without a divorce from Parsons.
I can't find any info about Harold Arthur having been adopted. I'm still searching for that piece of information but I am totally shocked that my Grandfather had a sister born in 1917 who he never knew about and now I'm researching all types of people in a haphazard way!
Why on earth would Elsie have traveled to America by ship every year?! What a life. She seemed to travel alone a lot but she's with Charles H Baber on one voyage, whom she later marries. No idea what happened to Ivan at the moment. I thought I was at the end yesterday but it feels like only the beginning!
Thank you all so much for your help.