Duane was seen using this guitar strap most notably during Atlanta Pop Festival in July 1970. At that time he played a Gibson Les Paul Goldtop, which is believed to be his third one in total.
This same strap was also seen on some earlier photos, taken in Jacksonville in spring 1969. It was used on another Goldtop, which differed from the Atlanta one by the “poker chip” – the white plastic cover around the toggle switch. This earlier Goldtop didn’t have one, while the Atlanta Pop Goldtop did.
That’s also the same strap Clapton used on the Layla album, its on the back cover with Brownie