40ac is formed of 2 words. Unsurprisingly given the theme, both words begin with the same letter. There aren't many options if you've got all the other answers.
hg - you must already have the first (and second) letter of 40ac if it's your last clue. That's the theme of the puzzle. As said in the preamble, individually the 2 words are thematic, so they both start with the same letter.
Thanks to peterm and Joan. I had a mental block there, looking for the whole word to conform to the theme, rather than each individually. It could confuse a stupid person.