From the USPostalBulletins.com website:
Postal Bulletin #8224, dated February 20, 1907:

It seemed to take quite of bit of defining and redefining during 1907:
Postal Bulletin #8233, dated March 4, 1907 (Note section 1e as the beginning of permitting a message on the divided address side), (sorry, poor image):


Postal Bulletin #8280, dated April 27, 1907:

Postal Bulletin #8345, dated July 15, 1907 (sorry, poor image):

Skipping forward ... the 1913 volume of the Postal Laws and Regulations was next compiled edition in hard-back form. Section 403.4:

Here front/back of a glittered card marked "unmailable":


And a postmarked glassine which likely contained a glittered valentine, postmarked at Louisville, Ohio, on February 13, 1909, the second day of issue of Scott 367.

And lastly, a 1914 Christmas card with stamped/postmarked glassine (Unsealed and with minimal writing so accepted as single-piece 3rd class, thus rated at 1 cent.) :

