I was reading up on the Japanese wiki page about this lens and it showed a few alternatives, so I looked them up and found the JML 56mm f/1.2 oscilloscope lens.
This lens was used by the US govt. when the Nikkor was no longer available. Seeing it was so cheap on ebay I purchased one.
804_1289It turns out to be a bit larger than expected, roughly twice the size, and the inner dimension of the rear flange is slightly larger than the outer dimension of the Nikon F mount so I am guessing it was designed for a larger film format than 35mm.
It has an internal electrically controlled shutter, which can easily be removed, but it also acts as the diaphragm (5 sided) the aperture of which is controlled by the knob on the right side.
804_1288As a result of the different design, magnification is substantially higher when placed up against the lens mount. As a comparison (different days, different flowers, but close enough) here is the JML 56mm f/1.2 first:
804_1270 Compared to the Nikkor O
803_0681I think it is useful for pictoral effects on smaller objects. Here are a couple of examples of how it renders the cherry blossoms in my front yard.
804_1281 and the wisteria which would not cooperate in the wind.
804_1272I don't think it has the same fine quality as the Nikkor, and the aperture is harder to work with, but for somewhere near 1/20th of the cost, it may be worth trying it out.