I'm stranded in Frankfurt airport for 5 hours waiting for my connecting flight to Geneva So to make good use of otherwise lost time I did some more digging into this factory letter mystery.

I looked again at all the companies that were part of the Union Horlogère in the period that Gruen was also a member of this joint venture and found the movement manufacturer Robert Frères, Villeret (from 1929 Minerva SA).

Given the letter logic for all the other confirmed movement suppliers, I'm pretty confident that we can assume that "Factory R" is Robert Frères, Villeret.

I recall that Robert Frères was previously suggested for "Factory D" however your movement find makes this assumption less likely to me now. There was however an ebauche manufacturer called "Duret & Colonnaz, Geneve" that was also a member of the Union Horlogère. Maybe this company is "Factory D"?