Typically, YES, that message would indicate no O/S was found..but also it could mean that the drive is not bootable.
I would check;
BIOS Settings for correct HD settings, and boot sequence settings.
If that doesn’t do it, boot with a known good floppy disk (Win95B or better for FAT32 support) and then check to see if you can see anything on C: drive.
But note, if these drives were originally NTFS, then a typical boot disk will not ID these either.
You’ll need to use a TECH TOOLS CD and boot off that with something like Winternals Disk Manager or ERD Recover tool to see what’s on those old machines.
Ever heard of BartPE?? Bootable CD with customizable tools, I believe there’s a file viewer on it. Do a search for BARTPE and you’ll find it.