I have some similar "contest" mail from the 1950s. Here is an example. Most of these were "legitimate" contests, and the covers made it out into the world because some collector knew the folks running the contest.
I ended up with several 100s of these, mostly different, for my Texas postmark collection.
An interest situation occurred in Israel during the 1956 - 1973 wars. An enterprising collector wanted to get examples of the many Israeli military unit markings scattered around the country. So he took out an advertisement in the military newspaper (comparable to our US Army "Stars and Stripes" in WW2) inviting the soldiers to send in the attached coupon for a drawing for a small monetary prizes. He must have received thousands of entries. Of course, all it cost him was the ad cost and the prize.
He opened one of the envelopes, declared the named individual the winner, the rest were sorted and packaged for sale.
Here is one sample envelope. The triangular mark identifies unit 1066. Many were also censored a benefit for the israel postal history collector.
Here is the Linn's ad I saved from a 1975 issue. Mine came from a different source, and I surely didn't pay that prices. Although today it might be worth that.