I read somewhere that the "HD" meant the servo could handle 7.5 volts. But the specs I find for this servo list a max of 6.
If you look at the datasheet: https://www.pololu.com/file/download/HD-1501MG.pdf?file_id=0J729 you will
notice its made by HuiDa RC International. HD stands for HuiDa.
Part numbers are usually (but not always) of the form