First let's look at post card rates during this period. The sender had a choice of air mail or surface mail conveyance. The air mail rate to all destinations was 10 cents and there were two surface letter rates, 6 cents and 7 cents depending upon the destination.
1. Air Mail to all destinations : 10 cents
Delta to Mali, September 26, 1969
10 cents international air mail post card rate
2. Surface
The surface rates were a only a few cents cheaper than the air mail rate.
a) Preferred rate
to Great Britain, the Commonwealth, the Republic of Ireland, France, Spain, and North and South America : 6 cents
Kitchener to Lisburn, Northern Ireland, May 25, 1971
6 cents preferred surface post card rate to the United Kingdom
Surface post cards to preferred destinations are scarce.
b) To all other counties : 7 cents
It isn't difficult to understand why the rate is challenging to find. Why would anyone send a post card by surface mail when for an extra 3 cents the card could zoom off to its destination. I suspect that people may have simply mailed the cards with the current 6 cent domestic rate stamps without giving the matter much thought.
So, what happens when a rate collector can't find a rate? Apart from continuing the hunt, short paid and unpaid mail can be an acceptable substitute (at least for me).
Over the years I have been fortunate to locate the following shortpaid and unpaid surface post cards mailed during the 7 cent surface rate period, November 1, 1968 to June 30, 1971 :
Niagara Falls to Warsaw, Poland, March 16, 1970
7 cent surface post card rate
6 cent stamp probably purchased from machine
Shortpaid 1 cent
Since the card was shortpaid 1 cent, the T 2/12 tax fraction was applied by the Canadian post office.
An explanation of the taxation system can be found HERE.
Windsor to Athens, August 31, 1970
7 cents surface post card rate
Shortpaid 1 cent
2/12 tax fraction
Toronto to Zagreb, Yugolavia, August 30, 1970
7 cents surface post card rate
Shortpaid 1 cent
2/12 tax fraction
40 dinars due
Powell River to Lausanne, Switzerland, May 17, 1971
7 cents surface post card rate
2 / 12 tax fraction
Swiss tax 10 centimes
Vancouver to Budapest, May 9, 1969
7 cents surface post card rate
Unpaid
14/12 tax fraction
Hungarian due 2 ft.
I don't know if I will ever find a 7 cent surface post card rate, but I'll keep on looking. There has to be one out there somewhere!