The image below is an enlargement x10 of the upper horizontal edge of the stamp appearing on the associated letter.
 

The image below is an enlargement x10 of the lower horizontal edge of the stamp appearing on the associated letter.

These enlargements show that the perforations of the upper horizontal edge were guillotined, but not those of the lower edge, which proves that the stamp comes from a booklet of 20 stamps.

The two above pictures are two photos taken under UV with an enlargement x10 of the stamp appearing on the associated letter. We see that phosphorescent bars are of type A ( no peripheral ring).