Hi Pentagram,
I wrote my return address under my signature for about the first 4 weeks or so. By then, I figured that my PP must have had it written down, somewhere. :-)
But . . I always put my PP's name, ID No. and address on the top left of the very first page of my letters. I just figure that "if ever" the envelope gets destroyed, and the letter survives, then they will know who to send it to.
My brother works in the post office, and believe me, envelopes do get destroyed. Some machines were invented specifically to do that. :-)

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