digeriukas,
It appears to me to be a Y.
With a normal watermark, the open spaces in the waffle watermark are slightly taller than they are wide.

With the sideways watermark, the opposite is true.

Yours appears to be wider than tall, which would make it a Y.
Also, there's a slight "grain" to the paper which is normally vertical. Yours appears to be horizontal, which would be consistent with the paper being sideways.
Michel 2016 CV is €2 for MiNr 241 used, €80 for MiNr 241 Y used.