I’ve always assumed that television viewers watching M*A*S*H on March 18th, 1975 — that’s when “Abyssinia, Henry” was originally broadcast, for those of you who haven’t memorized air dates — were completely unaware of what was in store for Colonel Blake at the end of the episode. After all, the bulk of the cast and crew had no idea that Larry Gelbart and Gene Reynolds had decided to have the character die after his plane was shot down until just before they filmed the scene in question. Based on everything I’ve heard or read over the year, it really does seem like most people truly were surprised by the shocking turn of events.
But just how much of a secret was Colonel Blake’s death? I started thinking about this last week while researching McLean Stevenson’s reported cameo appearance on The Carol Burnett Show. How could that series, which was taped in front of a live audience two weeks prior to broadcast, have included a segment in which Blake survived the plane crashing into the ocean, if nobody knew about the character’s death? Certainly, Stevenson and the rest of the cast and crew knew. CBS and 20th-Century Fox executives no doubt were aware as well. And the episode was filmed long before it actually aired, so there was plenty of time for the new to come out.
Earlier this month, Rob Kelly posted a scan of a newspaper article from the night “Abyssinia, Henry” was broadcast, which basically revealed that Colonel Blake would die by writing that “the ending is a la Mr. Roberts.” If you’re familiar with the play, novel or film Mr. Roberts, you’ll know what that means. The article was published on March 18th, 1975, the day the episode aired. So anyone looking at television listings and reading the summary for the episode — and also knew what happened to Mr. Roberts — would have had the episode spoiled to some degree.
I’ve found a March 17th, 1975 editorial in the Evening Independent (St. Petersburg, Florida) in which editor Robert Stiff writes about the season finale of M*A*S*H and McLean Stevenson’s departure. Here’s an excerpt:
Advance word is that Col. Blake will be discharged from the lovable “M*A*S*H” unit and die in a helicopter crash on his way home to civilian life. 
It’s not quite accurate but close enough to completely spoil the ending to the episode, more so than an allusion to Mr. Roberts. Stiff condemns the decision to kill off Colonel Blake, arguing that he doesn’t need to die in order “for viewers to be given the message that war is not a fun thing” . Interestingly, Stiff also has this to say:
I hope the good colonel departs the series without loss of life because I understand a non-fatal ending of the show has been shot just in case the producers decide to give him a reprieve. 
And thus, another M*A*S*H urban legend was born. No, there was not an alternate ending in which Colonel Blake made it home safe and sound. But now we have additional proof that in the days prior to the episode being aired, newspapers were writing that Colonel Blake might be or would be killed off. Imagine my surprise when I found an article from November of 1974 that revealed everything. Entertainment columnist Marilyn Beck, who at the time was syndicated by The New York Times, wrote the following in her November 13th, 1974 column:
“MASH” celebrated the completion of this season’s shooting with a cast party at 20th Century-Fox late last week. All the cast and crew were on hand — except for McLean Stevenson, who celebrated his permanent departure from the company by not attending. He left the soundstage as soon as the final camera “cut” was called.
Stevenson is moving on to a new contract with NBC, which he is convinced will bring him greater stardom than he has enjoyed as Lt. Col. Blake on “MASH”. As for how CBS will handle his absence from the series, they’re “killing” him off. One of the last things shot before the “MASH” company disbanded for hiatus was a sequence in which Blake finally gets his discharge — only to be killed in the crash of a plane bound for home. 
I can’t say how widely syndicated Beck’s column was in 1974 or how many people read it, but anyone who did knew exactly what to expect five months later when “Abyssinia, Henry” was broadcast. Whether anyone remembered reading about the details of Colonel Blake’s death after all that time is another matter. Also, Based on the publication date for Beck’s column, it is likely that the final scene was filmed on Friday, November 8th.