I've had a look through, but I can't discern anything obvious. As you suggest, it's not mono/stereo, and I don't think it's genre either. Price code would look the most obvious explanation, but I have nothing but deduction to work from on that.
Can any Welsh-speaking SAIN experts tell us what the difference between D and H is? I'm assuming Stereo / Mono? Later issues were H and earlier issues D but this says D & Stereo? Is it therefore an error on the label rather than an error on the sleeve?
Looking at Hogia' Wyddfa
their LPs went D, H, D, D, H. So there doesn't seem to be rhyme or reason in this letter unless it was linked to price or genre?