Entries are listed alphabetically by manufacturer/brand and organized by date added to the library. To quickly find a particular brand I recommend a search (CTRL-F or CMD-F), though browsing is kind of fun too.
A. B. Dick
11/13/2021
The Charles Deering McCormick Special Collections and Archives of the Northwestern University Libraries generously provided clean scans of the following documents from their Russell N. Cansler papers. Cansler (co)author of the two books on mimeographing taught at Northwestern for many years.
First is the 1957 edition of A. B. Dick’s Fundamentals of Mimeographing:
The second book is The Modern Mimeographing Handbook for Stencil Typists, Stencil Artists, and Mimeograph Operators.
05/20/2021
The A. B. Dick Standard Service Book of Instruction, 80 pages (including end pages and flyleaf), unknown date of publication (though there is an endorsement from 1919 just after the title page) is available as a PDF.

Banda
08/26/2022
Banda Model 10 and 10D spirit duplicator instructions, courtesy of Kevin B.
Copy-Rite
05/24/2021
This set of documents includes the dealer guide, operating instructions, maintenance guide, parts list, and schematics for the Silver Streak and Golden Streak fluid (spirit) duplicators (ca. 1959). Generously provided by Theodore Munk.

Gestetner
Operating instructions for the Gestetner 260.
10/22/22
Gestetner 460 operating instructions provided by Kevin B.
Operating instructions for the Gestetner 1120, an electronic stencil imager
Heyer
02/14/23
Thanks to Kevin B., I’m happy to host the Heyer Spirit Duplicator Master Service Manual. Kevin says, “this is an invaluable set of documents, and many of the parts were common to the stencil duplicators like the 1770 as well” – so perhaps it will be of use to the mimeograph operators who need to work “under the hood.”
Kevin has also provided the instruction manual for the Lettergraph Model E (which may be relevant to Models C and D as well).
10/26/22
Thanks to Kevin B. for scanning and sharing the following documents. First is the instruction manual for the Heyer Lettergraph Model 24:
And instructions for the Heyer Conqueror Models 1770, 1776, and 1777.
The following two parts manuals are for Heyer’s paper folders, the EF1 and EF2 – included here since they frequently accompanied the stencil or spirit duplicators of the same line.
05/06/2021
Instructions accompanying a Heyer Model 60 Postcard Printer (I guess once you attach the stencil it’s assumed that the rest is self-evident):

Rex-O-Graph
05/24/2021
The following documents were generously provided by Theodore Munk.
Rex-O-Graph’s parts catalog and price list (no date).
Fluid duplicator instructions (no date).
Roneo
07/05/2022
A multilingual (English, French, Dutch) user’s manual for the Roneo 350. Courtesy of Mimeograph Revival reader, Kevin.
Speed-O-Print
06/11/2021
The following documents were generously provided by Theodore Munk.
Four different parts-list publications:
1. Speed-O-Print dealer’s parts list and service manual.
2. Speed-O-Print parts list no. 14 (1962)
3. Parts list for duplicators, “speed-o-scopes” and cabinets.
4. Speed-O-Print photo copier parts list
A one-page parts price list.
Speed-O-Print Liberator 200 instruction book.
Speed-O-Print Model L instruction book.
Instructions for removing and replacing Speed-O-Print rubber roll assembly.
Standard
07/05/2022
Operating manual for the Standard Rocket Spirit Duplicator. Courtesy of Mimeograph Revival reader, Kevin.