is the clearsat model 3231 a dual polarity LNB?
It certainly is:
Dual polarity - horizontal (18V) and vertical (13V). Frequency range of 11.7 ~ 12.75GHz so will cover all services from any of the Optus satellites in Australia. This includes VAST and Foxtel.
Is this LNB suitable for an offset dish? What is the attachment clamping size or are they all the same?
This is an offset LNB designed for an offset dish (all small dishes in Australia used for Foxtel/Austar/VAST etc are offset). It will fit into a standard LNB holder as all are 40mm diameter throat.