I was going through some covers when I noticed this one from Dec 12, 1976.

It was returned to sender but also charged 8c postage due.
The domestic first class rate between Sept 1,1976 to February 28,
1977 was 10c therefore the letter was underpaid by 2c.
Since normally the charge is fixed at double the amount of the
deficient postage one would think that the postage due should have been 4c not 8c.
Am I missing something here?
Also notice the inverted
dec on the date slug.
