Interesting shot this one...
Some reciprocity failure - but the transparency is just perfect.
The shutter was open for a total of 60 seconds but closed 3 times during the exposure.
It was shot from Princess Bridge which carries heaps of Tram Traffic. The whole bridge shakes as trams cross, so each time a tram came close, I closed the shutter till it left the bridge.
One of the joys of using a camera with a copal shutter and manual film wind is that you can open and close the shutter as many times as needed.
This allows you to exclude Plane, boat and other traffic from otherwise congested skies and waterways. See http://www.pbase.com/image/133258738 for example.