When partitioning slice c of an RA81 disk under 4.3UWISC, you will get the following error:
newfs: ra81: no default block size for `c' partition
Despite /etc/disktab appearing complete, this error is unavoidable. In order to properly partition an RA81 run the following command:
/etc/mkfs /dev/rra0c 891072 51 14 4096 512 16 10 60 2048 t # special size [ nsect ntrak bsize fsize cpg minfree rps nbpi ]
This can be retrieved using newfs -v special when newfs fails in this way.
It is likely this could also be used to circumvent whatever issue is causing the write error: 759667 error when installing to an RA81.