Is this a Japanese or European plum?

I am in Melbourne. This plum grew by itself. Must've been from a store bought seed. It's around two years old. Initially I thought it was an apricot tree because it grew about a metre from my apricot tree (which is no longer there). It looks very healthy and is growing like crazy. It had one white plum on it this year. It was about 3cm by 2cm wide. I can't remember what colour was the flesh. It might not have been fully ripe either.

I want to graft two other plum varieties onto it.

Is this a japanese or european plum?