No Author can read (or find) every single book, paper, journal, etc., ever written on either subject. Somtetimes he/she have to rely on (trustworthy) friends, colleauges and simply belive what they have written/corresponded (in litteris).
A fact/name could surely have been published somewhere (and as such making it valid), in this context only forwarded/corresponded to the Author in litt. (in litteris), like (for example) what we´ve been doing/finding in the Bird Name Etymology subforum (where some additions/suppliers of "unseen" OD's or other unknown etymologies ended up as "in litt." in James's Key), while a fact/name in MS (in manuscript) indicate that a fact/name never have been published, but was/is (hopefully) about to be (in a forthcoming book/paper). Note that several in litt. is based on in MS.
A manuscript (in MS) is a work in progress, all the way till it´s been published, and as such it can change (as long as the Author of it is still alive). A manuscript is not a complete text, not to rely on (until published), belive me, my MS evolves just about every day. A letter is what it is. It says what it says.
And, of course, both could be wrong. Or the opposite.