50 Pfennig white numeral in circle, olive-green, single franking on insured letter for 20 Mark, single-circle "STUTTGART Nro 1 7 JUN 80 11-12V". Stamp well perforated, faultless, in original position, minimal insignificant browning beside stamp. Envelope sealed on reverse, overall very good condition, neatly opened at top and right. Correctly rated insured letter over 10 miles: 40 Pfennig letter fee and 10 Pfennig minimum insurance fee, total 50 Pfennig. Single-circle arrival "FRANKFURT A/MAIN 1 7/6 80 8-12N" on reverse, unsigned, Hartmut Winkler BPP photo certificate (2022): "Mi. no. 51 is well perforated and faultless", "Single frankings of Mi. no. 51 are extraordinarily rare". Michel cover catalogue 900.- for single franking, in our opinion still undervalued; our archive contains exactly 2 single frankings on insured letters, ex Postal Inspector Albert collection