In the Armoury section of the Core Rule Book, the Customised Weapon Special Quality is listed on page 115.

I am unable to find this quality listed against any of the weapons, and had presumed that it would be a weapon upgrade. However it is not listed there either.

Can anyone shed any light on this?

Or

What availability modifier do you think I should give it, if I use it as a weapon upgrade? My personal thoughts on this, is that it would be dependent on the weapon being customised.

I suspect that it got cut from the book along with torps, and the hunting rifle..... Personally I'd raise difficulty one step to get it.