When a Techmarine takes the Machinator Array trait, from what source does the pistol or melee weapon come from? I get the impression (that I can't confirm because I don't have the Rogue Trader or Dark Heresy books) that Machinator Array was taken from one of those games, where possessions and wealth actually accumulate, thus making the newly mechadendrite mounted weapons simple: You still had to buy it or already own it, now it's just welded onto a little servo-arm. (Whatever modifications are necessary to the weapon to make it mechadendrite mounted are made when you're getting the Array installed.)
But in Deathwatch, none of the interpretations I can come up with are satisfactory.
It can't just "come with" Machinator Array, or you can get a free power fist.
If you have to requisition it normally, then you have to return it to the Deathwatch armory after every mission...and can change the weapon at the start of every mission. Which both goes against the "cybernetics are permanent" policy and would indicate that the real upgrade you're getting is a little hand on the end of a mechadendrite that can hold a pistol or melee weapon, which is silly.
The best answer I can come up with is that you requisition the attached weapon as if it were a cybernetic, that is, you buy it on the next mission, but then keep it forever. But that seems kinda cheaty and is not at all indicated by the text. Help?