John, Shane Lovell has written an excellent booklet on the detail modifications of the Australian Centurions in Vietnam, although this is now out of print. You might be able find leftover copies of "Australian Military Briefs No3, Australian Centurions in Vietnam" through the publisher's distributors
http://www.mheaust.com.au/MHE/distributor.htm
Shane has also written an addenda to this booklet, but it is only online, not published as a second edition. This addenda is at
http://www.mheaust.com.au/Aust/Resea...entaddinfo.htm, and also includes a link to a discussion of Centurion turrets.