I am not quite clear on what your purpose is for doing this but let me point out a couple of things.
1) The first 512 byte sector contains the MBR, (Master Boot Record), and the MPR, (Master Partition Record). This 512 byte sector is often incorrectly called the MBR. Actually the MBR is contained in the first 446 bytes. The last 66 bytes are some flag bits and the MPR.
2) If you make an install and need to use your backup as you have done it, you will wipe out your partition table. It will be put back like it was before you made your new install, wiping out your new install in the process.
I would suggest that you use bs=446 for one file and bs=512 for another file. You can then choose to replace just the MBR or the MBR and MPR depending upon what you want to accomplish at the time.
I presume this floppy can also be used to restore the MBR and menu.lst.
If you have done what I think you have done, you will be able to restore your MBR/MPR, but not your /boot/grub/menu.lst. The menu.lst isn't resident in the first 512 byte sector.
Just a couple thoughts.