The style is very similar to other medals put out in the 1950s and 1960s, and very different to how a typical pre-WWII souvenir medal looked. The stippled background behind the portrait, for example, is an effect which is alien to Art Deco and earlier time periods. I think 1954 was chosen as being the only significant date with respect to Eureka within that window.
As for the anachronisms - the old spelling of Ballarat and the Queen's portrait - I would put that down to the medal-makers wishing to illustrate how things were back then. I should also point out that, although rarely appearing, the second "A" wasn't officially dropped by the Ballaarat City Council until 1996. Its use may even be an indication that someone in the Council commissioned this medal, but I'd have thought Mr. Carlisle would have found records so indicating if this were the case.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis