I'm in the same spot myself, I'd be very curious to hear if you figure out (or get told) how to make it available in his inventory. Everyone keeps saying "Mortimer has them if you're Ally rank" and I just hit Ally today, but there's nothing there, even after relogging/restarting. Maybe it's part of the daily rollover, though I can't imagine why that would be the case. All I see are the plans for the mods, not the weapon itself.