Hi from Bangkok. I am researching the same people, and am just now diving into the original records that HRN65 suggested in the 2011 note to you on the Vest Agder board. Those were some good hints, and the scanned parish records are helpful (especially when someone else provides direct links!).
I would acquire the Birkenes / Tveit bygdebok and look this up for us if I knew how to get a copy... any current sources?
Best wishes + looking forward to collaborating. +Bob