According to Mathscinet, the name of this journal was
A Magyar Tudományos Akadémia. Matematikai Kutató Intézetének Közleményei.

Abbreviation: Magyar Tud. Akad. Mat. Kutató Int. Közl.

It changed the name in 1977 and now is called Alkalmazott Matematikai Lapok.
But the paper you refer to is listed under the old name.