(the B-index column indicates individuals who may be found in that index, with corresponding reference number for further research)[/i]
From the wording (above) regarding the "B Index 339/3" notation on the passenger list cited, it seems to me there is a possibility of further information being available about them, held perhaps in the Archives - and possibly to do with them being Assisted Immigrants.
I don't know how you would go about finding out, but perhaps you could contact the SA Archives for advice?
http://www.archives.sa.gov.au/I also wouldn't focus too much on John being born "June 1814 and May 1815" by basing this soley on his recorded age at death.
Ages on death certificates can be and often were, wrong - sometimes out by a year or 2, sometimes by much more. At this stage, I would say you couldn't dismiss any possibile candidate born between say, 1812-1820.
Assisted immigrants also were apt to doctor their ages in order to place themselves within official or percieved age-criterias
Cheers
AMBLY