Saw your message while visiting Wpg and checked out a book written for the interlake region in the early 1980's called Wagons to Wings. I did check for the names that you had mentioned and although there were a number of Thorsteinson, and Peterson, Peturson and a Pjeturson, couldn't find the first names you mentioned. If you have not received any responses to your query, you should try to contact someone in the Lundar area, maybe they could help you. What about the birth dates or other family members' names.
Good Luck on your search.