Form 8-K
The filing that reports a material event, including a change in control.
A Form 8-K reports a material event between annual reports: an acquisition, a departure of an executive, a bankruptcy, or a change in control of the registrant. It must usually be filed within four business days.
An 8-K reporting a completed acquisition is the most precise possible evidence of when ownership changed, because the company is required to state the date and the mechanism. It typically names the acquiring entity exactly, which is how you learn that Unilever bought Ben & Jerry's through a Conopco subsidiary called Vermont All Natural Expansion Company.