I found 2 old amd64 floppy images from 2.0. No idea why I still have these:
-rwxr--r-- 1 andy andy 1474560 Nov 30 2004 boot1.fs
-rwxr--r-- 1 andy andy 1474560 Nov 30 2004 boot2.fs
Can you try writing a file of that size to one of those /dev/rsd files using dd?
(The man page for fdformat says 2006, I'm surprised it's that new.)
Andy