20528, from http://vehicleheaters.co.uk/Heater%20Matrix%20Make/Heater%20Matrix%20-%20Volvo.html
Price is about 103 GBP plus postage.
The list may suggest something else, but the 20528 actually fits the RHD 740/940 found in OZ.
The old ACC, manual A/C, and newer climate control units used in 740, pre-88 760, and 940 cars sold new in OZ all used the 20528 style, AFAIK.
I don't know what the post-88 760 or the 960 use.