You should really just take out the g1 lens, use it for something else, and sell this one at a reduced price.
While a great laser, it doesn't make sense to a potential buyer... 400mW of 405nm can be had for a whole lot less off the forum, and even less then that if self built.
You could also buy another lasers4u module & switch the lens half. He doesn't have a listing up for it at the moment but you can ask him.
The lens could be held in with a press fit retaining ring as the bare module was sold with it.
The module was also sold with g1 lens already installed by lasers4u.
It was listed as: 405nm Laser Module Housing, BARE- no lens Knurl type $8.99
Its 10.5mm dia, 19mm long w/ focus nut, 14.5 w/o. Attached is lasers4u's pictures (forgot I had them).