I found this antique on eBay for (somewhat) under HP’s 1980-era $35 price:
The prevailing price for HP 09872-60066 Digitizing Sights seems to be $100 and upwards, with outliers in both directions, so I just couldn’t pass it up.
Anyhow, the fiber optic pipe still works just like it did, back in the day:
The small dot in the middle is actually a paint-filled indentation on the bottom surface:
With the bottom flat on the target, the relayed image is in perfect focus:
The bezel recesses the top surface by 25 mil to protect the imaging plane.
OK, it’s a gadget gloat; I have absolutely no intention of ever chucking a piece of paper in the plotter and digitizing any points.
The Smell of Molten Projects in the Morning 
I did, so you don’t have to: http://scruss.com/blog/2016/05/17/pen-plotters-not-just-output-devices/
It was not really worth the effort, given what you can do in a web browser these days.s
Turns out I have a stack of “historic” hand-drawn dot plots that need digitizing; I thought I’d have to gimmick up something involving a scanner and OpenCV, because I could never endure the digitizing sight for that many dots.
Many thanks for the suggestion!
In a similar vein, there’s a device to optically position a center punch. http://www.leevalley.com/US/wood/page.aspx?p=45502&cat=1,180,42311
A 3D printed adapter for the HP sight springs, unbidden, to mind. [sigh]
I’ve gotten to the point where I either don’t care how close the hole comes to the target or I use the Sherline for coordinate drilling.