The 1908 Reading Pretzels featuring Home Run Baker. He debuted with the Athletics in September of that year. Not an RPPC but meets the other criteria.
This is the circa 1915 Portland, Maine Duffs coached by HOFer Hugh Duffy whom I believe is in the front row at right. Not a pre-rookie image but what the heck.
1926 Toronto Maple Leafs with 23 year old Carl Hubbell in the front row, still a couple of years away from the Bigs. Another HOFer in the guise of Lionel Conacher is seen in the back row at left. He is in Halls of Fame for hockey, lacrosse and Canadian Football.
Not an RPPC and no HOFers but I'm just itching to post it anyway. 1908 Richmond Colts featuring an essload of T210ers.
Don't have this postcard in hand yet. Won it at auction the other night. The AH claimed matter of fact that the gent at the very left is Casey Stengel with the 1910 Western Dental School ball club (? !). I don't think there's any way that it's Stengel but I liked the card all the same. If I'm wrong then I'll be stoked. Probably what we have is an RPPC of a pre-rookie dentist.