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I've always heard that Davo includes only one version of every stamp
No!
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leaving out most (all?) stamps with different perfs, coils, watermarks, and so on.
Different watermarks, generally, are included on the primary pages.
Their policy on perforations is unclear. Sometimes, they are included on the primary pages. Sometimes, there are additional pages at the back of the album. But they may ignore them.
GB has Machin phosphor bar varieties at the back. NL has syncopated coils at the back, but some issues with two-sided perforations are included on the primary pages.
They started out as a local publisher. Their cooperation with Stanley Gibbons made them a small international operator. They, since, have expanded. however, their main markets are the Dutch and Belgian markets, and, more recently, the UK. Their albums are based on the local preferences.
It makes no sense to design albums for a few specialised colectors. So you will see some depth but no true specialisation for the Germanies, France, Netherlands and Overseas, France, Belgium, and UK. Other countries will be more basic depending on the popularity in their main markets.
Individual pages can be ordered. They are made in Germany. You may send them an email inquiring after the existence of the pages.