Regarding Rudy Guliani 'M' tweet

MS-13

Members of Mara Salvatrucha, like members of most modern American gangs, utilize a system of hand signs for purposes of identification and communication. One of the most commonly displayed is the "devil's head" which forms an 'M' when displayed upside down. This hand sign is similar to the symbol commonly displayed by heavy metal musicians and their fans. Founders of Mara Salvatrucha borrowed the hand sign after attending concerts of heavy metal bands

https://en.wikipedia.org/wiki/MS-13

"devil's head" which forms an 'M' when displayed upside down.

https://en.wikipedia.org/wiki/Sign_of_the_horns