Postal Inspectors marked money

The inspectors, after reaching the post office department, plotted the routes of stolen mail and focused their investigation on a specific baggage car. To gather evidence, they prepared 15 test letters containing currency and recorded the serial numbers of the bills.

What were the inspectors able to plot regarding the stolen mail?

What specific location did the investigation narrow down to?

What did Emerson Bakers provide to assist in the investigation?

What type of letters did the inspectors prepare for testing?

What information was recorded about the currency bills?