Happy to help you out with the facts. I have Al Manns The Purloined Thought at hand, number 18 of 200 printed and signed to me by Al on November 23, 1990almost exactly 20 years since he gave it to me.
On page 2 of The Purloined Thought Al begins his history of the Center Tear by describing Sids experiences in England, his meetings with various famous English magicians (George Johnson, Davenport, Wheeler, Edward Bagshawe, Harry Leat and others) as well as his visit to Cambridge, his entertaining the Cambridge Rowing Club and how Sid met with John Gambling who Al described as entertainer to Royalty and the first magician to design and have built a locking flap slate in 1887.
During Sids entertainment of the rowing club he was invited to visit the exclusive Cambridge magic club for university students called The Pentacle where he met the secretary of the club, an American student, J.T. Garrus.
Apparently Al got this information from The Sphinx as Sid wrote two long letters describing his experiences in England that were published in the October and December 1928 issues. Sid made no mention in his letters to The Sphinx of the Center Tear. He did however write three letters to friends describing the Center Tear in detail. These friends were Tom Bowyer, Joe Ovette, and Ted Annemann as noted by Sid on the upper right of the first page of his notes.
On page 7 of The Purloined Thought Al reproduces in reduced form what you have reproduced on your blog in an enlarged version.
Al also reproduced the contents of a letter from Sid describing how Sid found a copy of Ovettes commercialization of the Center Tear that Ovette called What Is It? which he advertised for $1 in the September, 1931 issue of The Sphinx. Sid sent Al a copy of Ovettes publication signed: To my friend & Pal Sid Lorraine Oct 20, 1931. Al noted on the bottom, Sent by Sid Lorraine 4/14/78.
Al didnt think Ovette sold many as Ovettes gift to Sid was numbered 32A. As Sid wrote to Al, I didnt keep too many of Joes mns as they were always badly written and mimeographed and usually copies of someone elses material. Sid mentioned that he had chided Ovette for marketing the thing I had sent him from England.
There seems to be more contributions by Sid to Als research as Al reproduces a photo of Tom Bowyer that is signed by Bowyer to Sid. Sid also sent the information that when he returned to the US he tried to find a Mr. Wyman of Boston but did not learn of his identity. Als research discovered that he was Dr. Leland C. Wyman, at one point the president of Boston Assembly #9 of the Society of American Magicians. Al writes that Dr. Wyman was a fine magician and a serious student and investigator of spiritualists and mediums. Presumably Wyman learned of the Center Tear through his interactions with spirit mediums. Also interesting to note that Ovettes commercial use of the Center Tear was as a Living and Dead test.
I think it safe to say that Sid felt left out of the history of the Center Tear as neither Ovette nor Annemann bothered to mention Sids generosity in their published material. Sid also sent letters trying to set the record straight. In the Feb1966 edition of The New Jinx Editor Bill Madsen inserted a paragraph within an article by Leslie May with Sids memory of how the Center Tear came to be known in the magic community. Sid wrote that he performed it in England after he learned it from Garrus, I puzzled quite a few English magi with it, at the time. It was quite unknown among magic circles.
Sid also wrote a letter to Bascom Jones that was paraphrased in the October 18, 1974 issue of MAGICK.
These last two correspondences by Sid were reproduced in The Purloined Thought.
Al relates many other details about the dissemination of the Center Tear, who recognized it for its value and so forth, but that goes beyond Sids great contribution.