I own a handful of 1904 Conlons, the year he first began baseball photography. Below are the Walsh and Willis images with white borders on thin paper. While mine are unstamped, the Detroit Public Library Ernie Harwell collection owns two exact copies of my images. The backs shown with the Conlon 216 W. 111th stamps are copies I obtained directly from the DPL in my research.
web-Walsh-front-1-Net54.jpg DPL-Walsh-back-Net54.jpg
web-Willis-front-Net54.jpg DPL-back-Net-54.jpg
If you simply enter "Conlon" and "216 W. 111th" in the search box of the Harwell Collection at the DPL literally hundreds of Conlon images will appear that have research notes mentioning the 216 W. 111th Conlon stamp. As best I can tell the photos are all from 1904 to circa 1909.
I have never seen non circa 1904 Conlons issued in the approximately 4 x 6 sepia toned version shown above. As much as I love them, the 1904s all lack the clarity and artistry of his later work. It would seem improbable that a photographer would choose to reissue his "rookie", and only his rookie, images a decade or two later in an inferior format with his earlier stamp. The simplest, and to me most likely, explanation is that a young Conlon issued the white bordered sepia toned 4x6 images circa 1904 and placed his 216 W. 111th stamp on some of them during that period as well.