Ok, i have a solution. For now, i don't know why i get 15 when i do the following conversion below.
String input = "La Pe¤a";
Encoding extAscii = Encoding.GetEncoding(437);
Encoding win1252 = Encoding.GetEncoding(1252);
var bytes1252 = win1252.GetBytes(input);
byte[] output = Encoding.Convert(win1252, extAscii, bytes1252);
int problem = output[5];
so what i decided to do was to convert from asci to asci
byte[] output = Encoding.Convert(extAscii, extAscii, bytes1252);
int problem = output[5];
And that is how i get 164 which in ascii is ñ