Requirements:

Investigate 15 News Articles
Investigate 15 Reward Posters
Rewards:
Forgery Slot Upgrade, 3 Silver Crates, Platinum Crate
Notes: Finkerton's Boards are investigated with Eddy's Associates. News Articles are investigated with Railroad Stories. Reward Posters are investigated with Criminal Cases.
All three items can drop from Unwanted Rails (one each) and Fingerton Background checks (all three).