Too old to have had one as a kid, I did however find Stretch Armstrongs all the time at car boot sales. In the 1990's. Sometimes he was full dressed in his boxers ans vest, sometimes not. He was always stiff however and in need of a massage.
To unstiffen him I would place him along the top of a radiator wrapped in a towel. Being full of corn syrup, which hardens when it cools, the heat would loosen everything up again and give the figure the stretchiness he was famous for.
As a toy dealer years ago these loose figures always sold well but the postage was quite high. Old Stretch was weighty.
I never found a boxed version of the figure or his pet dog, Fetch Armstrong. Much harder to find again was the Stretch Monster. I wonder if there was a stretch limo too?
Did you have a Stretch Armstrong?