It does remind more of mid to late 60's styling, but early 70's would not be too far off either IMO.. Probably has an actual Ice Box and a hand pump sink too. Probably no actual heater as such.. Bet it has a propane lit lamp in it too! :)
I doubt there was an owners manual even when it was new.. My 1971 Ford LTD's owners manual was about 50 pages.. My 2013 F150 has over 500 pages in it's manual! That thing doesn't have anything in it that would need instructions on how to use! ;)
Nice find for your SIL and it should make for a nice project.
Mitch