This provides a nice case study of how a professional historian could shed more light on this incident without necessarily uncovering anything more about the incident beyond the yeoman's work of Mike C and Joe B.
For example:
1. Did Joe Dey write any of the Public Ledger articles? (I think he would have been writing for them at the time of these articles.) If so, did that help contribute to his reputation as a golf writer, which led directly to his becoming the USGA's first secretary? Is there any evidence of a dispute among the executive committee at the USGA, one possible outcome of which (eventually) was the hiring of Joe Dey? (More generally, is there anything to suggest what impact this incident may have had on Dey? For someone who nearly entered the seminary, presumably it would have made some sort of impression. Did he ever mention it? Did he make any decisions while with the USGA that theoretically could have been influenced by this incident? What about his tenure as commissioner of the PGA Tour?
2. Do the USGA archives indicate who was behind the decisions? What was the process -- did it involve a few individuals, an entire committee, etc?
3. Is there any record to indicate how the leaders of the USGA behaved outside their golf-related duties? How did they run their businesses? Did any court cases or union / labor issues surface that may provide some insight?
4. What was the general state of race relations in the country, in Philadelphia, and in NYC society in 1928? Despite being far, far more restrictive then than now, the climate still could ebb and flow. And important regional, even city-specific, differences existed. An African-American's lived experience could vary tremendously from one region to another, one state to another, and one city to another. What's the larger context into which this episode might fall?
5. What's the record say on African-American participation in this event in other years? Was this a "one and done?"
Mike and Joe, sincere thanks for the effort!
Mark
PS Something interesting about the Shippen "incident" is what role CB Macdonald may have played in the decision. Macdonald was Shippen's playing partner, presumably a sign that Macdonald supported and perhaps was actively involved in the decision. And here's the twist in that tale: according to his grand niece, Macdonald was "part Mohawk Indian." To my knowledge, nowhere else have I seen that fact reported. (Assuming of course his grand niece is correct.)