letter of transmittal - сопроводительное письмо, но, допускаю, что transmission letter имеет совсем другое значение

препровождения письма/записи о передаче материалов дела

Letter of Transmittal
The letter of transmittal “announces” the arrival of the report. It can be in letter or memo
format (usually the norms of the organization will dictate which one), and should be
addressed either to the person who has been your primary contact or to someone in senior
management.
In a sense, it is “ritual communication.” It may briefly remind the reader of the
assignment that was made and then lay out the scope of the report. It may give the
conclusion of the report (in no more than one or two sentences). It may also suggest next
steps. It should thank the reader for the opportunity to work on the project.
The letter of transmittal should be no more than a page, and often it is shorter.

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Note added at 1 hr (2017-06-06 16:56:39 GMT)
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https://ocw.mit.edu/courses/sloan-school-of-management/15-27...

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Note added at 1 hr (2017-06-06 16:57:25 GMT)
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http://www.linguee.ru/английский-русский/перевод/transmissio...

Оно же: transmittal letter --> http://universal_ru_en.academic.ru/401619/transmittal_letter

Письмо, которое отправляется вместе с документом или ценными бумагами. Оно содержит описание и цели сделки …
http://translate.academic.ru/передаточное письмо/ru/

См. также: https://www.google.ru/#newwindow=1&tbs=li:1&q="передаточное ...