Hi Tony:
You beat me to it!
The unusual spider rims had me fooled. I dug around last night and learned that Canada and The UK used the Martin-Parry adapters to convert 13 and 18 pounders as well as 4.5" hows from wooden to pneumatic wheels. Australia used a home grown solution which resulted in the different looking wheel arrangement.
Had me fooled! The gun I thought it was is the Scoda 100mm but once I got to looking at the pictures, the characteristic profile of the 4.5" how became obvious.
I'd like to learn more about the Aussie adaptor as it looks like it allowed for using a much wider tyre than the Martin-Parry which was quite tall and skinny. Thanks for the other photos, they're great!
Mike