1967-6-26 / 550 / 700 / 10 grams
Franked 2 x 350 which was the foreign 20g letter rate so they'd have them in the office. 350 surface + K255 200 5 grams air = 550 so 150 overpaid. Mind you, 200 was the inland letter rate so they'd have had them too! Franked at 10am on Monday - perhaps posted in end-of-week rush the previous Friday and delayed because the checker didn't work weekends...