AFAIU, the sale of printed manuals was one of the earliest income streams in the Free Software/open source community, and for GNU/FSF in particular. O'Reilly and maybe some of its contributing authors (some of whom were project maintainers or contributors) made appreciable money selling printed documentation. But calling any of those examples a "business model" for open source development is a stretch.

The sale of support, OTOH, absolutely was an early business model. See, e.g., Cygnus Solutions, Red Hat, etc. And today many project maintainers make money in a similar manner, through contracts or outright full-time employment related to feature development and integration. But this happens just as often in BSD land as Linux land.

The GNU project does sell print copies of their texinfo manuals. But you could also download the manual and print it yourself. Buying the print is an optional way to support the development of the software.