ARRI is a German manufacturer of high-end cinematographic equipment, currently offering state of the art digital imaging tools. This is a Type E projector, No.274, so old and obscure that information about it is very hard to find
It belonged to my grandfather. He was a teacher who earned a little extra money duplicating educational filmstrips. He was thrifty and clever, adapting what he had on hand to his needs, like this early ARRI piece.