See UEB 5.7.2. What you are really doing is ensuring that a letters sequence is not incorrectly read as a word in braille. So with Hm you aren't uncontracting the word, you are making sure that the letter sequence Hm is not read in braille as the word Him. For that purpose, the grade 1 symbol indicator is sufficient and takes less space. This applies to CD, LLC, the al in al dente, and so on.