Well, panzeriv, I am not sure I have all the information and can guarantee they are all correct. But all I believe is:
The Mk 3 uses a 20-pound main gun(80.3 mm). The Mk 5, if I am correct, for the early version it also uses the 20-pound gun(80.3 mm) but with a fume extractor. Later Mk 5 are upgraded to 105 gun(also with fume extractor).
The Australian Certurions in Vietnam were most likely Mk 5. Anyway, they had the fume extractor on their main gun. The Modelcraft Centurion Mk 3 I mentioned
has NO fume extractor. One way or the other you have to remodel the main gun with the fume extractor. Also the Mk5 has no rear turrel escape hatch.
Check these sites for futher references:
http://www.lancers.org.au/Museum/Centurion.htm#Aust
http://www.diggerhistory.info/pages-armour/allied/centurion.htm
and these two sites for pictures for Ausralian Centurion in Vietnam
http://members.tripod.com/CenturionTank/OtherCents/index.html
http://members.tripod.com/CenturionTank/OtherCents/page1.html
I know I still might miss out a lot, can anyone help panzeriv more? And also let me know if I make some mistakes. Thanks.