By chance I saw a triangle mark on the $2 Singapore note contrasted against the other $2 notes when paying for lunch. So out of curiosity I compared the marks and voila it seemed we have a number of different variations for such markings. The markings are just below the word 'EDUCATION'. I saw those $2 notes with squares before but today was the first time I noticed the triangle (top piece amongst the 3).
I guess this is not unique to the $2 notes, cos' when I checked the $10 notes, the triangle mark is there as well (2nd piece amongst the 3 notes) below the word ' SPORTS'.
Wonder what next, will I find a note with circle instead of a triangle or squares? Not sure if there are notes with two triangles since there are $2 notes with two squares.