"conveyance from one place to another," 1724, from transmit + -al (2). Originally interchangeable with transmission, but that word now has a second sense.
Entries linking to transmittal
transmit v.
"send over, onward, or along," c. 1400, from Latin transmittere "send across, cause to go across, transfer, pass on," from trans "across, beyond" (see trans-) + mittere "to release, let go; send, throw" (see mission). Related: Transmitted; transmitting.
-al 2
suffix forming nouns of action from verbs, mostly from Latin and French, meaning "act of ______ing" (such as survival, referral), Middle English -aille, from French feminine singular -aille, from Latin -alia, neuter plural of adjective suffix -alis, also used in English as a noun suffix. Nativized in English and used with Germanic verbs (as in bestowal, betrothal).