It would seem that login reward drop chances are not released (if anyone can find something to the contrary, I'd love to know!) At the very least, login rewards do not seem to appear in the official DE-maintained drop-rate list.
As far as we can tell, login rewards seem to be a random choice from a pool of rewards (excluding every 50th day, which has a set reward, like you noted).
The Wiki states that "the chances of more frequently acquiring rare and higher value items increases as players increase their Mastery Rank," but I am unable to find any source for this.
Also anecdotally, people seem to report better chances of a high platinum discount after taking a week or so off. I'd imagine DE is very interested in not making that public, if that's the case.