Ok, I used to have:

> SPARCstation 20 (1 X 390Z55), No Keyboard
> ROM Rev. 2.19, 160 MB memory installed, Serial #7493664.
> Ethernet address 8:0:20:72:58:20, Host ID: 72725820.

with much thanks to Rob Windsor I now have the right eprom, 
and I've installed a pair of CPUs so now we get:

           SMCC SPARCstation 10/20 UP/MP POST version VRV3.45 (09/11/95)


CPU_#0       HyperSPARC ROSS RT620/RT625         0x00040000 Bytes ECache 
CPU_#1       HyperSPARC ROSS RT620/RT625         0x00040000 Bytes ECache 

CPU_#2       ******* NOT installed *******
CPU_#3       ******* NOT installed *******

...
...
SPARCstation 20 MP (2 X RT625), No Keyboard
ROM Rev. 2.25R hyperSPARC, 160 MB memory installed, Serial #7493664.
Ethernet address 8:0:20:72:58:20, Host ID: 72725820.
...
...
NetBSD 1.4.1 (SS20) #0: Fri Sep 10 23:43:12 EST 1999
    root@ss20:/u0/NetBSD/NetBSD-1.4.1/src/sys/arch/sparc/compile/SS20
real mem = 167288832
avail mem = 153477120
using 1792 buffers containing 7340032 bytes of memory
bootpath: /iommu@f,e0000000/sbus@f,e0001000/espdma@f,400000/esp@f,800000/sd@3,0
mainbus0 (root): SUNW,SPARCstation-20
cpu0 at mainbus0: RT620/625 @ 100 MHz, on-chip FPU
cpu0: 256K byte write-back, 64 bytes/line, sw flush: cache enabled
cpu at mainbus0 not configured
obio0 at mainbus0

I'll install the rest of the CPUs once I can utilize the first pair.

--sjg