Advertising cars traveled to locales ahead of the circus and carried personnel that prepared for the pending arrival of the circus. They typically traveled on the rear of whatever passenger consist was traveling to the desired destination. For example, if the circus was scheduled to visit Philadelphia after a Pittsburgh visit, an advertising car would travel to Philly a week or so ahead of arrival at the end of a Pennsy consist.
Of the two cars that MTL has released to date, the mail-baggage is from 1890 and the parlor is pre-WWII. I've seen pics of the real parlor, so I can vouch for the accuracy of that model.