Q. Can you use an old get certificate of divorce (ptur – the Beis Din is not extant, but they were reliable Rabbanim) to prove that the man or the woman named there went trough a proper act on conversion?

A. See Piskey Din (Rabbanut Yerushalaim 1 p. 52) in regards to establishing Jewish identity via a valid kesubba. However the case of a p’tur or divorce certificate may be different, since it is possible that the Beis Din granted a get lechumra, and they were in doubt of the Jewish status of the divorced, yet acted stringently to remove all doubt of divorce.
Horav Shlomo Miller’s Shlit’a opinion is that the case should be presented to a competent Rabbinical Authority, since there could be other variances and issues.

Rabbi a. Bartfeld as revised by Horav Shlomo Miller Shlit”a