The article "Making Sense of M101-5 & M101-4" by Tim Newcomb and Todd Schultz that appears in issue #16 of Old Cardboard is extremely helpful in understanding the scarcity of the different back advertisements found in these issues.  The Block & Kuhl backs are among the most rare, a small Illinois department store that distributed cards from both the M101-4 and M101-5 issues. This example featuring Chicago's Hap Felsch is one of fewer than 60 examples from the entire issue that have been graded by PSA and SGC combined, and the only graded Block & Kuhl example of the outfielder, banned from baseball for his involvement with throwing the 1919 World Series.  Perhaps a unique example from an unbelievably rare issue, the card is accurately graded yet boasts eye appeal in excess of the technical grade.