I don't have an actual Debian machine here, but the same mechanism should work.
jinx:1467 Z$ dpkg-query -S /etc/debian_version
jinx:1468 Z$ dpkg-query -l base-files
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Description
ii base-files 5.0.0ubuntu20.10.04 Debian base system miscellaneous files
jinx:1469 Z$ _
So, as a first cut, you should be able to depend on a specific version of the
base-files package. You can map releases to version numbers on http://packages.debian.org, but in this case you already have the versions: they're the same as the Debian releases (5 vs. 6; ignore point releases for this comparison).