Madrid arrival marking

[Jaime Benavides]

M N 5  -  I believe it is an arrival date stamp  i.e. MADRID NOVEMBER 5th (not positive about this but see attached image as reference)

[Martin "Spuff" Spufford]

 
I have to hand this undated Colonial cover sent from Mexico to Madrid.
Its somewhat outside my scope of knowledge.
The Mexico "NUEVA ESPAÑA" cancel is dated in Yag and Bash as 1805.
I cannot find anything on Spanish arrival markings on Google.
Does anyone know if the marking "M" "N" "5" (see scan)
means maybe
Madrid November 1805?
 
The pen "9" is that in reales? to pay.