I was able to use a few of the coupons from the 40th Anniversary game and for each one I received an email "Seat coupons applied to your trip XXX-YYY". It contained the coupon number, record locator of the flight, and assigned seat. Of course it could go to spam so a screenshot's still a good idea.

These coupons seem like kind of a gimmick but I was able to get real value out of them. My flights were nearly full and most of the good seats were already assigned 2+ months out. If I had waited until T-24 when they became free as a Gold I probably would have missed out. First was full or nearly full too so the chance of people being upgraded out of the prime seats was low.