Gents -
On the subject of it being an auto, the (previous owner) made the comment in passing that it was originally an auto but perhaps you can confirm whether this is the case.
While trawling through the forums I came across a record of all the HS30's accounted for in Australia. It was very comprehensive.
The record indicated that the vehicle produced just before or after one was an auto (HS30 01424 ??) but for the life of me I can't find the spreadsheet again to confirm. That is some supporting evidence.
Secondly, the centre console has been modified to accommodate a manual transmission, which you can from the photo on the Gumtree ad. http://www.gumtree.com.au/s-ad/townsville/cars-vans-utes/1972-datsun-240z-coupe/1135792440
Is the centre console standard for both the auto and manual, or are they different moulds? perhaps the shape gives it away. Unfortunately I don't have the depth of knowledge to make this judgement.