Quote:
Speculation only, but in the mid-1970s 11c, 13c and 17c were at various times the one ounce rates for domestic airmail and the 13c also for regular first class mail.
I thought the same until I checked the 1st class rates at the times these stamps were issued. The 11c & 13c were issued in early Jan of 1978 ... the 1st class rate at the time was 13c. So your speculation could apply to the 13c postage due stamp. The 17c was issued in 1985 when the 1st class rate was 22c. 17c was never the 1st class rate.