Judging by the picture I am certain that it is the OLD standard swedish "Stora BM" quick coupler you have there. Its narrower at the top.
Most likely the original one that came with the machine. Its possible to rebuild it or order a bucket that will fit.
Some blue prints on the coupler:
Stora BM maskinsida
CAD ritning på Stora BM fästet
/EDIT: This is a pic of my machine with the newer(up to date) "Stora BM" that is wider at the top. It can attach both old and new tools: