This is a rare find and can be resold by the yard quite profitably if you are so inclined. Each spool originally contained 76 yards. The black one is nearly full. I estimate the white one is 75% of more full. I have seen this sold by the yard for $6.00. I am trying to downsize my extensive sewing collection and time is of the essence for me right now.
Elastic button loop tape is used for wedding dresses and other projects where you need closely spaced / decorative closures. These still have elastic stretch despite their age.
My estimate is these are from the 1940s. They came from the estate of an antique dealer who was also a seamstress.