David,
This photo is scanned from "Kubinka, The Russian Museum of Armoured Vehicles" by Michael K. Cecil and show one of these that was sent to the Soviet Union for testing.
As far as I know a total of 129 was built, 100 was delivered to the British Forces, 26 to the Canadian Forces and 3 to the Soviet Union Forces for testing.
Is it possible that SM 2561 only includes the 100 vehicles delivered to the British - the chassis serial at the one I have seen is DND 100 - 80, the other one in the same location have chassis serial DND 100 - 81. DND is possibly the Canadian Departement of National Defence through which the order (SM 2561) maybe was placed? Maybe the Canadian order have a LV (or CDLV) contract demand number. And the ones sent to the Soveiet Union?
I will attach some more pics as well
