Now you mention it the original fade-out did stop rather suddenly, although it had faded quite low by that point so it wasn't too jarring at normal volume. Whereas the 08 remaster has a more gradual extended fade, including a few seconds of run-out that didn't feature before.
It makes me wonder how they actually ended the track in the studio - did the different instrument tracks each come to a definite end as per live performances or just peter out? I thought the 86 ending without the volume-down was much better than previous live ones with the 'on-stage fadeout' which always sounded a bit lame to my ears.
23 minutes of SR plus near 2 minutes of Horizons is pushing vinyl to the limit IF you want to keep the bass at proper levels - bass gives wider grooves in the vinyl, and there is only so much room between the records edge and the run out groove, which on many turntables will activate the autostop.
A cutting engineer will be aware of these various limitations that need balancing, and it's even possible they did 2 or three versions to be presented to the producer and band, to see how they wanted to have it released.
CD avoids all this problem, they can keep it going as long as they choose, up to 80 minutes total.
I'm not aware of any Genesis tracks that have been released to include previously unheard starts and ends, but I have a few Moody Blues ones, and it's quite interesting to hear the full track. There are even some with comical bits, The Story in your Eyes starts with a voice (Graeme Edge to my ears) saying "it's a marriage of classical and Rock" for no obvious reason. Question ends with Justin doing a little random acoustic guitar solo, and Gypsy ands with a mayhem of instruments, chimes, and a strange laugh, which all fades out then returns briefly. So, bottom line, there could be all sorts on the master tapes, Steve has alleged that there is a 3 part guitar harmony on SR at the end which was never mixed in. A little odd if so, for that is not something Steve has ever really embraced, though he clearly has the ability to do it. In contrast to Brian May and Justin Hayward who did it a lot, to name but 2.