This was a common procedure on the 748 in Scotland, with BA, when I was there.
We would load, maybe the front 4 rows or so, with mail and/or newspapers and the pax would sit in the rest of the cabin.
We had special seat covers to prevent dirtying the seats, there were various rules to obey and instructions about how to do it.
We, the pilots, had to load and unload the freight ourselves in the islands.
Sometimes we had boxes of lobsters, loaded thus, from Kirkwall to Inverness and sacks of malt for the Highland Park Distillery from Aberdeen to Kirkwall..
Great days in the early 80's!!