Those orthoptics are a very old-fashioned solution to eyesight problems, but they are still usable. Parker's used to still have a small stock but somebody swooped in and cornered the market in them.
Another 'cheat' is to use the metricated slide with the index mark machined off and re-inscribed lower down that was intended for the L42A1 iron sight. Of course you have to disassemble the sight to do this.
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