Apparently, when CN downrated some RS-18 to 1400hp and equipped them with A1A trucks for use on lightly built branch lines, they called them... RSC14. Since this was MUCH later (the first such rebuild was apparently in 1975), I'm inclined to think this is not relevant.
On the other hand... There was perhaps some idea, in the mid-1950s, that RS, RSC, and RSD roadswitcher model numbers should be in a single series. (Since, at east for RS and RSC, a unit could be converted from one to the other by simply swapping trucks, there was potential for confusion if the same number was used for different models in different series.)
And a few models were numbered this way. MLW's full-height hood variant of the RS-3 became the RS-10 (and also the Dl-700): I assume (in the absence of documentation showing otherwise) when it was introduced in 1954. The 251-engined successor to the RS-3 (introduced in 1956, but doubtless under design a good deal earlier) got the next available numbers in both the RS series and the Dl series: RS-11 and Dl-701. The (slightly longer and a good deal heavier) CC version became the RSD-12 and Dl-702: next in each series. Produced before them, but perhaps not designed or authorized for production until after they had been named, MLW gave CN the RSC-13. It was basically a minimally updated RS-1 on A1A trucks, so it didn't get a 700-series Spec number: it was the Dl-800. (CN later removed the centre axle from the trucks on at least some to produce a BB version: MLW offered to build new BB units, calling them RS-13, but nobody bought any.) So the next in both the RS series and the Dl 700 series would be our mystery model, the Dl-703 a.k.a. RS-14.
Its not the RS-18: Kirkland's book says that was originally called the RS-11M, before MLW (trying, I suppose, to look a bit more independent of its corporate parent Alco) gave it the (system-destroying!) numbers RS-18 and Dl-718.
So I'm left with two guesses. (1) A first sketch of an export model, abandoned in favor of the Dl-540/RS-16, or (2) the RS-10S or something like it: the major change in control system being, plausibly, enough to justify a new Spec and Model number. In scenario (2), perhaps MLW decided not to change the model designation because they made the change in the middle of a CP order, and didn't want to have to rewrite a contract that called the locomotives to be built RS-10.
If we are very, very, lucky, someone will find an archived document giving us another (and maybe more exciting!) answer.
(All based on a rereading of the relevant sections of Kirkand's book.)