I spent a long time Googling before finding this, only to see that it had already been posted. When I was in college I went on a tour of the Netherlands with my school's choir. We sang in a number of Protestant and Reformed churches and I remembered one of the music students pointed out a hymnal, still used by a congregation in Friesland, that used this particular notation system.
Someone else suggested that this was a notation with which they were familiar, from an old hymnal of the (U.S.) denomination affiliated with the college I attended. I had assumed it to be quite an old system as the hymnal appeared quite antiquated. Still, it doesn't surprise me that it (only?) dates to the 1930s, nor that there would still be churches using it. Many old Reformed churches have short institutional memories but nonetheless cling to assumed traditions. That, and the Frisians are, stereotypically, an old, stubborn people.
Someone else suggested that this was a notation with which they were familiar, from an old hymnal of the (U.S.) denomination affiliated with the college I attended. I had assumed it to be quite an old system as the hymnal appeared quite antiquated. Still, it doesn't surprise me that it (only?) dates to the 1930s, nor that there would still be churches using it. Many old Reformed churches have short institutional memories but nonetheless cling to assumed traditions. That, and the Frisians are, stereotypically, an old, stubborn people.