Talk: Line Printer ASCII Art

A long time ago (in a galaxy far away :-) there were ASCII Art files
designed for line printers.  They took advantage of the printer's
capability to overwrite lines on top of one another (the first
column/character could be used as a control character and not printed:
a + in column 1 caused the current line to be overprinted on top
of the preceding line).  This allowed a much wider grayscale than is
possible without overprinting.

For example, I have some old Mona Lisa printouts that look really nice
(well, not compared to the original), but I don't have the files
any longer.  Does anyone know if these files still exist somewhere?
Thanks for any help.
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