I have a 65 7ECA and have not seen anything like that in my plane. I have looked inside the tire fuselage and wings.
from the curve of the lower edge in the photos and the notch, it looks like it might fit vertical in the fuselage with the curve following the lower covering and the notch is for the bottom stringer. Maybe some kind of a custom bulkhead. Just a guess however.
Good luck.
ps: if you don't have the parts manual for the early Citabrias, you need to get one. Univair has them and I believe Wag Aero does also.
Dale