The images are DiskCopy (type:rohd, creator:ddsk). The reason you'll need 4.2 is because 6.3 requires System 7.0.1 or higher, which is unable to mount the old 400k (MFS) disks.

If you scroll down to the end of this page there's a link to a project that adds native read-only MFS support to Tiger. Might be worth a try!