Some here who have participated in JW related internet discussion for a few years might remember "Friend."
He was an anonymous JW, (His real name was known to only a few) someone of note in the JW organization who had access to letters and other research material at Bethel. He very much desired reform on the blood issue.
"Marvin Shilmer," Alan Feuerbacher and "Farkal" can all corroborate this. Perhaps others can as well. He posted both on H2O and here on JWD for a couple of years. (I believe his posts are still in the archive.)
"Friend" and I exchanged a fair amount of personal correspondence. His opinion, after researching the origins of the blood doctrine from the "inside," was that the primary impetus behind its development was the large number of letters JW's were sending in inquiring about its propriety during the second half of the 40's
He also said that one JW in particular, a Canadian lawyer by the name of W. Glen How, who is also someone of note in the JW organization sent in quite a few letters advocating the current restrictive position toward transfusion. He felt that How's letter writing campaign strongly inflluenced Brooklyn.
I realize this is only hearsay, and I offer it only for whatever interest in might hold.