Above is perhaps the first Thunderbirds-related piece of merchandise, and certainly the first appearance of a Thunderbird in TV21. It's an ad for Century 21 Records from TV21 No.41, dated 30th October 1965.
Here are a few other ads from around the last few months of 1965, when the popular things were Lego, Daleks, fireworks, and James Bond. Not much change there, although fireworks would not be advertised in comics now. Personally I think it's time for a revival of Stingray Slippers! (When I was 3 or 4 I had a similar pair with Yogi Bear heads on them! They were all the rage you know.)
As always, click on the images to see them much larger.