For lack of a better analogy, US History textbooks are, generally speaking, Schrödinger's source. What makes a textbook shift from being a primary to a secondary source is less the textbook itself and more the question you're asking. So, as you suspect, an AP US History textbook/course packet/syllabus from 1956 would be a primary source if you're looking at what content ETS prioritized when sending documents out to schools. They remain primary sources if you're tracking how AP presents content throughout history (for example, documenting how the language around chattel slavery has changed over time in official AP documentation.)

I'm not able to think of an example that would shift them into being secondary sources given the framing you've described, so I'm going to rudely tag u/Kugelfang52 who has done lots of work around the history of US History in American schools.