Century Arms built more than one, but I'm not sure what the number was. Might have been a pair?

The one advertised on ozgunsales is listed by Zero Sports Manufacturing in Springvale, which was founded by Fred Klimek. It would not amaze me if Fred had had a lot to do with building these guns, and I believe he's still around. Should still be contactable through the shop, which his son Anthony runs now [Anthony's diving shop shares the premises].

There are some detail photos on the Century Arms website, http://www.centuryarms.com.au/docs/4bore.html

Edit: Dive shop website [shares phone number with The Shooters Shop] http://www.springvaledivecentre.com.au/