What is is an "M7" Diode
It's a SMT version of a 1N4007 (1A 1000V PIV). They definitely exist (I have a reel of a similar type kicking around somewhere) and they are very popular in Asia (typically from suppliers that are lesser known in the West). 

You can use any 1A rectifier of sufficient voltage rating that comes in a DO-214AC (SMA) package. 



Edit: 

I had a look at the schematic- the purpose of that diode on the input is to short out a reverse-polarity input. Diodes are pretty rugged and tend to fail short, so unless the source is something like a car battery or PC power supply, there's a good chance it won't fry the board. 50V rating is more than enough for this part- the NCP1117S is only good for 20V in (abs max).