Although only 1 type of watermark had been used, there are 2 orientations of that watermark - each having 2 positions.
A symmetrical paper wire and an orthogonal watermark, the wire side is at the front of the stamp, so we are dealing with a FT or filigrana traspuesta! No coating. In other words - a mate nacional...
There is a noticeable variation in colour shades of brown ochre!
with a watermark that has the direction of paper running parallel to the long side of the stamp ("M") and the line of AЯ perpendicular to that (orthogonal)!
The S. Oficial prints in black have always been applied simultaneously with a stamp design itself. It is hence, bicoloured offset-litho! The direction of paper is, however, turned 90 degrees, and runs parallel to the short side of the stamp:
A less common variant has the direction of paper like the S.O. stamps:
