The prestamped envelopes were issued to commemorate the first anniversary of the declaration of Latvia's re-independence.
The flap of the 15k envelope bears the inscription "Valid for dispatch within Latvia,as well as to Azerbaijan,Belarus,Georgia,Estonia,Kazakhstan,Kirgiz,Russia,Lithuania,Moldova,Tajikistan and the Ukraine",but the flap of the 50k envelope bears the inscription "Valid for dispatch to Denmark"
(from the specialized catalogue of Latvia)
Here are pictures of envelope # A4
Those envelopes were reprinted twice.Reprint II may be recognized by the flap of the envelope that bears no inscription.