It isn't so much about pre-war laws or interpretations of them
It is exactly about the 1920 citizenship law, it's contemporary interpretation by Ministry of Interior's 1925 administrative ruling, Donald Tusk's 2009 "procedural" law which created a presumption that all of those Poles living abroad who failed to recognize the legitimacy of communist state had lost their citizenship, and how they all relate to the provisions of the present constitution prohibiting discrimination and involuntary loss of citizenship. A person who has never undergone the process for confirmation or loss of citizenship is unable to competently advise anyone on any of this.
Your best source of advice is your local Polish consulate who are used to handling enquiries about citizenship and immigration
From what I have seen posted here over a course of time, the consulates have routinely ignored the "military paradox" rule in the 1920 citizenship law, i.e., advice was routinely given that citizenship had been lost due to naturalisation, military service, etc., despite the fact that military service was not completed in the fatherland. If the consulate is unable to secure production of the old passport records for the OP's great-grandfather, they won't help with anything else other than taking his/her money. No one claiming citizenship by descent from the 1920 citizenship law should entertain the process without consulting a competent lawyer about that law, present practice, etc. If a lawyer doesn't know what the "military paradox" rule was, find another lawyer.
You used to work for me before you had to leave Poland. Remember?
I've never had to leave Poland. I've never worked for jon. He is quite confused. I don't think he could afford my services. I would remember working for someone constantly posting online while wearing a tin foil hat and gossiping about people he never met.
Thousands that are eligible for citizenship.
No, millions abroad were born with citizenship from the Second Republic. Under the best of circumstances, it takes years to find the supporting documents and get a favorable decision. Unlike the German model of repatriation, these people are not welcome to return promptly as full citizens. That is the problem.