Re the Horseman converter, I have one. Used as intended it is a 2x TC, doubles the focal length and the extension required. Users' reports about how well the TC + prime lens performs disagree.
It works as intended only with 150 mm lenses in #0 shutters. The instructions list a moderate number of 150s that will work with it.
Three parts. The first is a replacement for the shutter's retaining ring. The second is an extension tube that attaches to the replacement retaining ring. And at the rear the TC, which screws into the extension tube. To set it up, unscrew the prime lens' rear cell, remove the retaining ring, reattache the shutter to the board with the replacement retaining ring, replace the prime's rear cell, attach the extension tube and finally attach the TC.
The combined focal length -- prime lens + TC -- depends of the prime's FL and on the distance between the prime's rear surface and the TC's front surface. When used not as intended the results are quite unpredictable.
Not what the OP wants.
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