I tend to think the informant is Pierce.

On page "D" of the report, the informant refers to a 1956 meeting when Giancana was made boss of the Outfit. The informant doesn't reveal that he was at the meeting but he supplies such precise details-the location of the meeting, the agenda, the attendees, etc, that it seems to me that he was probably one of the guys at the meeting. (Someone could have learned the information secondhand but it would be hard to keep the details straight fifteen years later).

As for suspects, if we eliminate the dead mobsters and assume the informant isn't Accardo, Giancana or Alex, it leaves Kruse, Vogel or Pierce.

Furthermore, the same informant knows confidential details about Alex's current schedule that only one of his top subordinates-Kruse, Vogel or Pierce, was likely to know.

Combined with clues found in other FBI documents, I tend to go with Pierce as the source.

I wrote more about Pierce here.

http://informer-journal.blogspot.ca/2011/07/roemers-men-in-outfit.html