I did find the type of mail generally assigned to each class, but that's not exactly your question: second-class (newspapers and magazines), third class (bulk advertisements), and fourth-class mail (books and media packages).
I also looked for the first year machine cancels were used, but no luck. There's also the question of postal markings on drop letters, but that's way outside my knowledge. I have several examples of a Boston cancel like yours, but only on a stamp, not a letter.
Well, thanks Doug. It does answer the question of the class. I think considering the senders and possible contend, these would have been both 4th class mail.
I don't know if you remember the image of the photocard of a young woman named Hattie, which I has posted a while ago, and which had been sent through the mail, but it also had the same Chicago postmark as shown above. Guess she's 4th class as well - though that doesn't seem like a nice thing to say about a young woman I've never actually met!