The missing letter is:
Because what's represented here is:
the 'upper right corner' of the 'home' screen of this website:
The letters are the initials of the words which appear on the key buttons shown on-screen. 'A Q' is 'Ask Question', 'A' is 'Active', 'H' is 'Hot' and 'W' is 'Week'. What's missing is the 'M' for 'Month'.