Honest, all, I think I've done my best ...
Q/ How are souvenir photo folders rated?
- it is not rated as a letter, as the 1/2c rates ended in 1885;
- it is not rated as a postcard, as there never was a 1-1/2c rate (that I could find);
- it is not rated as a periodical, as it is not, well, a periodical;
- it is not rated as BPM (Bound Printed Matter), as that required an imprint;
- it is rated ... ???
if you'd like to see the whole booklet