The eyes are round and of equal size.
The arms of the cross are asymmetrical.
The ring at the centre of the cross is not round and the
lines are uneven.
The inscription starts with an x, which is unusual. Erik
forgot the T-rune in the fifth word "stain" (eng. stone) and
had to squeeze it in later.
Below, you can see an article in Swedish on U 755,
taken from the book Upplands Runskrifter.