Do it like Cheap_Suit has shown in the thread Drekes posted.
At the top of FM_SetModel and CurWeapon hooks, check for the weapon the way I posted.
Code:
new weaponname[ 36 ]
get_weaponname( CSW_KNIFE, weaponname, 35 )
new weapon_id = find_ent_by_owner( -1, weaponname, id )
if( weapon_id != your_desired_weapon_id )
return PLUGIN_CONTINUE
// other code