W1MK 1931 ARRL Headquarters Station, Hartford, CT

The W1MK QSL Card was folded in half to fit inside a biz envelope.

Early hamshack of ARRL HQ Station W1MK

The above QST page has a picture of "RB Parmenter" - the operator who signed the QSL "card" - he is the single image in the upper-right.

W1MK QSL Card + Article courtesy of W0EAJ
Article from QST Magazine
Photo courtesy of NL7XM