Some of the early models of the 39mm Leica thread EL Nikkors were made with very short thread length. I suppose that the Komuranon lenses might have used the same short thread length standard. Unless the lens board is quite thin, there might not be enough thread passing through the top side of the board to properly grip a retaining ring. I found this to be the case with an older 75/4 EL Nikkor.

The threaded shank of the lens was only 1mm or less above the top of the lens board. Given that and the starting chamfer at the beginning of the thread, the retaining ring would turn no more than 2/3 turn—far too little thread for safe mounting. I had to obtain a lens board with a 39mm flange and screw the lens into the flange.

The newer “N” version EL Nikkors have longer thread length and fit the standard Beseler boards with enough thread above the top of the board to grip a retaining ring.