Thanks for the pointer, John. I tried that before (btw. see my comments under "monkey-jsun" name at the end. ;0)
The CD-ROM part does not work at all. The net installer still goes out to the network and fetch the latest release and install.
To prove my points, I ran with debian 10.2 net installer and mounted a debian 9.5 CD. In the end it is the buster that gets installed.
BTW, the main reason I asking for CD/DVD install is that, after a while old net installer are not available anymore. And even if you dig them out from archive CD's, they don't work anymore (probably due to some bits not available online). That prevents me from re-producing some old bits and potentially fixing some bugs there. If anybody knows how to work around this, I'd appreciate it.