Well Brian, I don't have experience with that wire retainer, but Jack's comments were an interesting read.
The only 4 speed Mustang I owned was a 66.
Without the retainer, the wire has no support downstream of the firewall retainer like you say.
If there's an engineering number on it, I'd expect it to be on the black plastic portion.
specialed probably knows since he was the impetus behind the reproduction.
I guess there's so few nice used 69 boots and wire retainers around since everybody wanted a Hurst shifter back in the day which wouldn't end well for the 69 lower boot.