...spokeswoman said the two parts are "very difficult to distinguish, visually."
I hate to dogpile on, but building otherwise identical bolts suggests the switcheroo was intended all along.
The engineering world has a maxim called "Murphy's Law", which goes "If anything CAN go wrong, it WILL".
This is not an expression of pessimism. It is an instruction to the engineer, that he MAKES SURE to minimize the number of ways that things could go wrong. Like making electrical plugs intended for different voltages and power levels physically different, so that a plug CAN NOT be placed in a wrong outlet. Like making sure a part CAN NOT be inserted backwards or upside down.
In this case, the engineers failed in their duty to ensure that the wrong type of bolt COULD NOT be mistakenly installed where it wasn't supposed to go.