Postage Due Puzzler - Why So Much?

I've been working through an accumulation of covers circa Vietnam era, mostly Naval, and I cannot figure out the reason for the postage due on this cover.

The cover is franked with a Scott 1283 George Washington 5 cent stamp from the Famous American series, period use, but underpays the rate by one cent. This is noted at the bottom of the cover by the straight line postage due marking. Why then was it charged 25 cents?

The only thing I can come up with was that there were other covers bundled to this business and this was the top cover, receiving the postage due paid for all of them.

Anyone have any other ideas?

Here's the cover. The reverse side is not shown as there are absolutely no markings on it.

I agree, a typical "top of the stack" example.