I have 90' left out of 100' of 35mm out of date by at least 30 years. It is called Kodak SO-2575 'high speed direct duplicating film'.
It has an ei of about 0.12 now. I have no idea how fast it was when new.
It is a document film- ie really high contrast - but I have calibrated it to suit duping continuous tone material using home compounded low contrast and dilute commercial developers pictorial results.
I use it mostly to dupe old negs before I try stuff like intensifying or reducing them to try to improve thier printing quality.
Mostly this applies to old negs I shot before I knew what the proper use of development was, or what my wife's grandfather shot 50-70 years ago. Some of these old negs are horribly dense, and others are too thin from poor development.
I use a slide duper bellows rig and macro lens for copying 35mm material and a 35mm/4x5 Polariod technical copy camera with an old spare Omega D dichroic head inverted on the baseboard to backlight larger negs.
I also have a fair bit of 100' of 35mm old Kodak 'fine grain positiive release' left. It is currently rated at about EI3-EI6.
Fine Grained is kind of like kodabromide enlarging paper on film stock.
It too can be used to yield continuous tone work, but is not as high contrast as the SO2575.
It is not as straight lined in its response as the SO2575 is for providing a good quality dupe neg.
These direct positive films are pre flashed at time of manufacture, and you are exposing past the shoulder of the highlights, to get the direct direct positive result.
The third dupe film I use, though not for direct negatives is Technical Pan. I fluked into a 30year old full 25 sheets box of 8x10 of this stuff last month for $5.
I cut it down to 4x5 to feed it into a 4x5 Polaroid copy camera that I have. Regretably on about one out of every 4 4x5's I cut there is a gelatine adhesion problems, likely from being stuck to the sheet next to it somewhere warm for a long while.
Even with this annoying defect, it is a very versatile film that with a home brew low contrast developer gives a good linear response that is good for duping by neg duped to positive, and then contact printed to an enlarged negative again. It does very little to add its own grain to the duped neg.
Your should hunt high and low for an 80's era Kodak book called 'Copying and Duplicating'.
It can teach you a lot about copying neg's even if a lot of the films mentioned in the book are out of production.
This book recommends Plus X pan as a viable dupe film as well. I have not tried TMax100, which Kodak recommended as a sucessor to Plus X (at least I think that Kodak dropped Plus X).
As another aside, a 31 step step wedge and use of a lab's desititometer really help in getting your duping skills under control.
PM me if you want to negotiate for some of my more obscure stocks, and I will spool off a couple of 35mm cassetes of what you might like to play with and mail them on to you.
Payment by way of Paypal in $Can or $US is fine with me for covering the costs.