I believe it can be established from the NOBO list. it will show who owns what and when. Or, if not the company can request a dated proof of purchase of shares. I had this request once. I needed to send transcript copy or possibly a screen shot of your purchase.

With that said; one might receive additional restricted shares or a retro active dividend or immediate dividend? I'm not sure how the later would work.
In any case it is a very nice gesture on the part of the company.

I don't believe I will qualify since I have only owned shares of the company the past 60 or so days. I think "long" qualifier may be as someone with 6+ months of ownership. Also believe it could be based on amt of common shares held.