boot VaxStation 4000/60 from disk

Hi,

after I got a serial cable working, thanks for all of your suggestions, I 
was able to install OpenBSD via network (MOP).
A dmesg can be found here:
https://www.l00-bugdead-prods.de/VaxStation.dmesg.txt

Due to limited disk space, I only installed bsd, base43.tgz, and etc43.tgz.
After the usual "Congratulations, your system is successfully installed..."
I rebooted. In the chevron prompt I told the box to boot from hard disk, but 
this fails, with the following error message:
?43 FILESTRUCT

I assume I have to prepare the disk in a given way, to be able to boot from 
it? Or do I have to enter special parameters to the boot command on the 
chevron prompt?
I know from my old SGI Indigo, I have to tell the box, that there is a 
different operating system than the "standard" one coming, to be able to 
boot NetBSD, maybe  I have to do sth similar?
I also did not fiddled around with disk geometry or sth. like that, when I 
installed OpenBSD onto the disk, maybe I should have done sth?

The box itself seems to work fine, I can boot a NFS root filesystem via 
network, but it refuses to boot from disk.

kind regards
Sebastian