Hi Jon!

> think the Bendix G-15 had cassettes for the 5-level tape 
> they used.

Aha, interesting! Did a short search, but have not been able to
find a picture of a casette. Just a pile of paper tape instead ;-)

https://images.app.goo.gl/HYqkpYHJUxZeGfiA8

> of mylar tape instead of paper.  OS boot tapes might be punched 
> on that.  

Yes, for heavy use (and e.g. humid environments), the mil guys obviously
used Mylar or heavily oiled tape. Although not experrienced myself, I was
told that the Mylar tape cuts through the guiding pins of the readers over
time. And yes, it also may easily cut one's fingers ;-)

> much like plain paper tape, and simple mylar alone.  The latter often 
> comes metallized on one side, and is glossy.

Yes, that is the one within the casette and the aluminum as you expect 
is for optical reading (Most plastics including Mylar are quite 
transparent for IR light and in the old days, when thungsten bulbs
where used, the IR part of the light was the major part contributing 
to the response of the photodiodes!

> input medium for the university mainframe computer (Electrologica X8), 
> they used optical readers rated at 2000 characters per second.

Wow - that is indeed pretty fast!!! My FACIT is 1200cps maximum and 
stopping "on character" as it is called is VERY hard at that speed.
But buffering helps here and in case of the 920M, the casette is 
used for, there was no buffering. So software needs to be read in 
one run.

Best wishes,

   Erik.


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