I've read Snapshot and rollback of apt-installed packages and What does reading database mean in Apt?. My task (IMHO) is simpler than 1st question (for which there is no accepted answer): no need to roll-back, I envision saving enough of apt / dpkg state to allow to temporarily substitute saved files to be able to use apt-get install --download-only
from saved state.
What files (database?) are needed to be backed up and replaced? Any clearance / fix consistency commands needed after such substitution?
Via this procedure I expect to get needed packages for installation on a offline computer.
apt-offline
package, a safer alternative than manually mucking about with apt files.