The Dolby 70mm process went through a number of changes, with many co-existing at the same time.
The first Dolby 70mm system was format #40 in the Dolby format code directory (*). Five channels across the screen with one surround channel combined with roll off curves at the low and high end. Almost identical to the 70mm theatrical layout of that of the 1950s and 60s as used by Todd-AO and others.
Format #41 added Dolby A noise reduction and ditched the more aggressive filtering.
Format #42 added what were called baby booms, later referred to as low frequency effects tracks, put in place of the mid left and mid right channels. True five channel up front mixes had become exceedingly rare by the mid-70s, while low frequency effects had become popular at the time thanks to the Sensurround process, and Universal wasn't allowing it to be used by other studios without a hefty price tag. So in steps Dolby. (There's actually a great back story on the creation of format #42).
Format #43 added split surrounds, but the layout was an interesting space saving process. As the mid left and rights had already been designated as baby booms in format #42, Dolby opted to share the two baby boom channels with the split surrounds. The split surrounds would be filtered in the low end, as the low end of those two channels continued to feed subwoofers, via dedicated low end content, per format #42. And a sixth mono surround track still went along for the ride, providing mono surrounds for 70mm theaters who had not upgraded to split surrounds. Split surround equipped theaters were rare at the start and took some time to build a bigger base. Many 70mm theaters never switched over.
(*) The Dolby coding itself is worth noting as they of course wanted theaters to play each print correctly. And with the projectionist having said information, and 70mm itself being considered a classier event.. this helped reduce running a given print in the wrong format.