Thanks to Kenn Thomas for this email and scan:
The panel you used was from Not Brans Ecch #3, October 1967. Attached is the revised pnael from Not Brand Ecch #10 (“The Worst of Not Brand Ecch!”), December 1968. Martin Luther King was assassinated on April 4, 1968. Apparently it was still too soon to have his image in a satirical comic book.