I would call that a magnifying cursor.  It might be easiest to find one as a spare part for a table saw.  For instance:  https://sawstop.3dcartstores.com/Fence-Position-Indicator-Lens-for-T-Glide-Fences_p_63.html

Peter

I don't find magnifying cursors all that useful. They magnify everything. I prefer finer scribed lines.

My top preference is fine lines and a vernier scale so you can directly position to a fraction of an increment.

Google vernier scale, but the basic formulae is that you make a short scale that is either smaller or larger by one increment than the long reference scale. Then you simply match lines.

My old Paralok table saw fence was my introduction to this method. (The Paralok was designed by an architect)
The rail that the fence slides on has a long scale adhered to it like most systems. On the T bar is a small piece of aluminum with 8 increments on it. (4 to each side of the "0" centerline). The 4 increments span exactly 5/16 of an inch so when the 0 is lined up with an increment on the long scale only the marks on the outermost ends of the vernier scale also align with marks on the long scale. The other marks vary from 1/4 of a sixtenth to 3/4 of a sixtenth. So, moving the fence so the next closest mark lines up with a mark on the long scale means you moved the fence precisely 1/64". Moving the fence only enough so two marks equally stradle a 1/16" increment means you moved the fence 1/128".

I just added the Wixey digital readout to my table saw - very pleased with the results.  Accurate to 3 decimal points and you can calibrate by either sliding the fence up to the blade or cut a piece of wood, and directly enter the measurements into the unit

https://www.amazon.com/gp/product/B072HFQLBS/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

example pic from their website

I see your point, Michael, but I thought the primary purpose of that device was not to improve accuracy per se, but make the standard level accuracy available to those with less-than-perfect eyesight.

+1

Well to be frank, it’s called a cursor, a view finder, etc. But I’ve always heard cursor. Now the one you would probably want, depending upon your application is the Delta/Biesemeyer Magnifying Cursor #1352593S. I make a lot of my own fences for table saws, and these are the best in my opinion. I order mine directly from M&M Tools in Salt Lake City. https://www.mmtoolparts.com/store/ Good luck.