Try

	boot 3/tftp
or
	boot 3/mop

after setting up a bootp/tftp or MOP server with a kernel, and
NFS server with a root partitino for the now-diskless machine.
The NetBSD/pmax webpage has instructions on how to do this for NetBSD.
It should be simple for an experienced sysadmin to adapt those
instructions to boot Ultrix, if you have another sytsem to act as
 a boot server.

The PMAG-DV is the baseboard MAXINE display, which is supported by
NetBSD, so you should have no trouble there.

--Jonathan