They mixed real shark footage next to the prop.

Come off it, it’s easy to say that now, some 45 years later! Can you imagine how amazing it must’ve looked to audiences on release. I think it still looks good now.

I saw the movie back in 1975 and there was never a moment when the mechanical shark wasn’t real. I don’t think anyone distinguished between real and mechanical because the story was so good - and the terror was so overwhelming.