I'll take a wild stab regarding the 3853 which is one of the 10 design Presorted First Class Golden Eagles with water activated gum. 3853 has a red eagle on a gold background.
Is the stamp mint or used?
If used, it's possible that the wording on the stamp is 'Mailer's Postmark Permit', which would have been applied by the user. I don't know why it's not visible in normal light. Perhaps the ink was somehow bleached out from soaking, exposure to light, or other chemical reaction and it's the residue that's showing up under UV.
See this discussion for examples of mailer's postmarks.http://goscf.com/t/31250