Strange transistor driver circuit

Hi,
I've just been reverse engineering a printer controller PCB and I've come
across a rather unusual (to me anyway) transistor driver circuit in the
printhead solenoid driver.
Forgive my naff ASCII art, but here's the schematic:

+V +V
--- ---
| |
| 3 Solenoid
| 3
e | 3
b | / PNP +--|>Z---*
>-----[ 4k7 ]------|< | | c
| \ | b | /
+-----*-----|< NPN
| \
| e
|
---
GND

To me, that looks like a pretty normal complementary Darlington circuit. What
I'm most interested in is the 20V Zener diode on the NPN - ZD.cathode to
NPN.collector, ZD.anode to NPN.base. AFAICT, when the PNP switches on, the ZD
will get put straight over the solenoid, with the anode more positive than
the cathode, causing current to flow A-K to ground. Surely that would blow
the Zener to bits (it's a BZX84C20 - 300mW SOT23)?
If it was oriented cathode to +V, anode to NPN.collector, I'd have guessed it
would have worked as a rather expensive spark-killer/back-EMF suppressor. In
this configuration I'm not so sure...

Thanks.
--
Phil. | Acorn RiscPC600 Mk3, SA202, 64MB, 6GB,
philpem@xxxxxxxxxxxxx (valid address)| ViewFinder, 10BaseT Ethernet, 2-slice,
http://www.philpem.me.uk/ | 48xCD, ARCINv6c IDE, SCSI
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.... Noah! Come quick! There's water in the basement!
.