Very good account. But a few factual errors i.e. not Santa Claus on the postcard - see your postcard picture later and not a speedboat - either a ferry or hydrofoil.

https://www.pinterest.co.uk/pin/802133383617141103/

But I'd like to know the source of your new info that wasn't in the podcast or on NRK sites. Did you access the Bergen archive or have access to other copies of archive sources, such as the BBC?

One new historic/case context I found just yesterday was German serial killer Arwed Imiela was in Oslo in 1970 he killed on Fehrman island where the Beate Uhse sponsored concert took place in 1970. https://de.wikipedia.org/wiki/Arwed_Imiela

"Since the suspicion against Imiela's fiancée was not confirmed, she was released after three weeks" so 2nd week May 1970. Could she be the Isdal woman - said to be 27.