@steveroberts, I am afraid the Zootierliste.de information is most definitely incorrect on purported Sierra Negra Galapagos tortoise Chelonoidis viking. First of all: the data on species and sometimes their numbers assigned to species-level are inaccurate. Secondly: Not too long ago the site had undergone a complete overhaul and quite a bit of the historical information just vanished and Thirdly: Some of the contributors have updated listings with species/data/informations that are incomplete or even faulty...
NOTA BENE: Don't get me wrong ... I do think Zootierliste.de is a great resource ... lacking our access to ZIMS ..., but at times now I have for the above reasons a LOVE/HATE relationship with Zootierliste.de.
Having said that here I present some of the fullest historical data available:
Originally, the Sydney Zoo received 0.0.6 Galapagos giant tortoise in 1928/29. These were youngsters/larger hatchlings and a long way off from becoming mature individuals. At the time, and given the difficulty with sexing tortoises at an early stage their sex / age ratio remained a complete unknown.
In 1971 the Sydney Zoo acquired a male Galapagos tortoise hatched at San Diego Zoo - named Tommy - who was reported to be a Western Santa Cruz Galapagos tortoise Chelonoidis porteri and has since passed away (edit: in 2002).
In 1974 the then Sydney-Taronga Zoo maintained 2.2 Villamil Mountain Galapagos tortoise Chelonoidis guentheri ..., while a few years later in 1981 the zoo just 1.2 remained ..., so 1.0 individual must have passed away in the timeframe in-between. No mention of any other Galapagos tortoises...at this time!
The next import of Galapagos tortoises in 1991 was the larger shipment of 3.3 hybrid Galapagos tortoise from the Honolulu Zoo. These were born between 1967-1970 and constituted the F1 offspring from a mix of several species of Galapagos tortoise also originally imported from the islands themselves.
Effectively, in 1991 Dubbo WPZ had 3.3 hybrids and 0.2 Chelonoidis guentheri as well as 1.0 Chelonoidis porteri (M Tommy). Over the subsequent years 0.2 hybrid tortoises died (1997; 2005) plus 0.1 Chelonoidis guentheri (deceased in 2002) and the 1.0 Chelonoidis porteri (also deceased in 2002).
By 2003 Dubbo WPZ maintained 0.1 Chelonoidis guentheri and 3.1 hybrids.
Conclusion: While Zootierliste.de numbers may have been accurate, their life histories nor their correct species assignments have/had been applied in revision. My take on it ... if you do not do your research properly ... you end up with another Wikipedia hoax

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! So, be careful what you eventually put to paper (and I admit ... I have made mistakes ..., yet I do try and learn from each one of them or excuse myself ... with the audience / forum).