Mark and Derek,
If the March 31st date is incorrect, then the "Mother of All Primary Sources," H&W's own ledgers, are incorrect as well. As far as I've been able to find, there is one document belonging to PRONI -- some sort of ledger kept by the City of Belfast -- which has the 22nd as the keel-laying date. Every H&W ledger I've seen, and of the excerpts I've seen reprinted from the same indicate March 31st.
Mark, have you found any mention of this event in the NYT? More importantly, how about the local Belfast papers? Their readers would have had far more than a passing interest in any such activities at the shipyards, especially since the announcement of the beginning of every newbuilding meant anywhere from another several months to several years of worry-free employment for so many of Belfast's men and their dependants.
Regards,
Scott Andrews