Here is a Neostyle envelope designed with the hooked side flap for inspection purposes - with the flap opened, and a close-up of the embossed lettering on the flap-edge. The top flap appears to be sealed normally.
Here is a slightly later envelope with the patent dates on the side flap:
The two patents seem to match:
July 18, 1911 for patent 997,999 for an "Envelop", and
July 2, 1912 for patent 1,031,521 for a "Postal mailing card", although this one does not match as well, I do not find a better match for this date.