Specifically, Shank, Minor states:
"When using a light melee weapon with which he is proficient, the slayer may replace its normal base damage with 1d6 (1d4 if the weapon is Small, 1d8 if it is Large), and its normal threat range with 19-20."
Improved Critical (and Keen) then state:
This effect doesn’t stack with any other effect that expands the threat range of a weapon.
Would Shank count as expanding the threat range for this purpose, or would 'replacing' the base damage and threat range be akin to using a different weapon (essentially treating any applicable weapon as a 'Shank' - a weapon with 1d6 damage die and 19-20 threat range) as opposed to changing the damage die and threat range of the weapon itself?
As an example: Davey the Slayer uses Shank (the Talent) with his Hook Hand +1. Is the Hook Hand now considered a Shank +1, a weapon with 1d6 damage and a threat range of 19-20?