When I heard about this accident I delved deeper into it. The result was an account of a person who had been close to the site and taken photos of the remains. Unfortunately (and as usually) I do not have the link anymore as the matter was settled for me. :-( This is what I recall from the article:
In this account it was stated that the failure indeed happened to the front of the boiler. The barrel and front flue sheet were corroded excessively. It appears that the boiler barrel ripped horizontally from where it is riveted to the flue sheet. Thus the barrel was peeled back and the flue sheet blasted forward.
Hope this helps.