My father was a highly skilled tool maker and this sight arrangement has all the hall marks of the tool makers craft.
I still have my dad's tool chest with dozens of engineering gauges he made himself over the years.
As a side note....
In the early 1960's, the telecommunications company he worked for at the time lost a big contract to a competitor, pictures were 'obtained' of this rather complicated piece of equipment and my dad led a team of engineers secretly reverse engineering and bench testing it from the pictures alone!
Now that's an engineer!