For some bus stops the journey API does return stop letters like “->W” instead of “W”.
For an example take the journey from “Great Portland Street” to “New Cavendish Street” by bus.
https://api.tfl.gov.uk/Journey/Journeyresults/1000091/to/1010198?modes=bus
“arrivalPoint”: { “$type”: “Tfl.Api.Presentation.Entities.StopPoint, Tfl.Api.Presentation.Entities”, “naptanId”: “490G00010198”, “platformName”: "", “stopLetter”: “->W”, “icsCode”: “1010198”, “commonName”: “New Cavendish Street”, “placeType”: “StopPoint”, “additionalProperties”: [], “lat”: 51.51987524954, “lon”: -0.14386107355 },
I’ve also found this for other bus stops, for example “Abbott Road”