I am no expert on the history of Besser bricks (which I had never heard of until you gave them a name) as according to the internet Besser bricks are Australian so we probably don't have them under that name here.... but I have no idea. This picture is American so whether they have the Australian Besser brick I couldn't tell you.
Bricks have been made for at least 5,000 years so making a mould would not be at all difficult 100 years ago. I had a search on the internet regarding the availability of cement/concrete and that was most certainly available 100 years ago. Indeed, there are examples of its use of a type (1 part lime to 4 parts sand) existing from as far back as 300BC. Even as the product we are more familiar with today, it was a common product in the mid to late 1800s and the first concrete reinforced bridge in America was built in 1889 so yes, they did have this type of brick 100 years ago.
That said, I can offer absolutely no idea one way or the other whether these particular bricks have been in situ since the place was built or whether old timber has been replaced with them since then. I guess we tend to think that a lot of things are more modern than they really are. It is of course common that a product might be new in one country but have been in existence elsewhere for a long time.
Who knows?
Cheers,
Rob