I went on a bit of a Sherlock Holmes on this topic last year - lots of people saying 78 onwards but no advertisements to back it up - 1980 was the earliest hard evidence.
I imported a 1978 mongoose survivor a short while after that with prety much all era correct (although not stock mongoose correct) parts on it and it had a blue 2-piece DC clamp possibly bringing the 1978 theory back in to play
I'm doing two very different 79 builds next year and both will be getting DC 2-piece clamps on them and I'm happy that they're era correct enough for those
On another interesting note they were still available in 1986 and possibly later........
I have a 1986 dia-compe catalogue with them in :