Well, there were no DSLR's when these gadgets came out, in fact I'm not even sure that there were any SLR cameras, what was in common amateur use was 35mm rangefinder cameras, which in relative terms were very small and light.
From memory, these clamps sold very badly, probably because it was hard to find a convenient table, old gatepost or tree to fix them to, especially as most cameras had non-interchangeable lenses back then, and zoom lenses didn't exist.