[tz] why do Africa/Juba and backzone/Africa/Juba differ?

[Michael Deckers]

    Puzzles me in 2017c:

    Africa/Juba and backzone/Africa/Juba differ
    in their astronomical longitudes:
       in zone1970.tab:  SS  +0451+03137  Africa/Juba
           in zone.tab:  SS  +0451+03136  Africa/Juba
    and (correspondingly) in the local mean time used
    before the switch to zone time in 1931:
             in africa:  Zone  Africa/Juba  2:06:28 -  LMT 1931
           in backzone:  Zone  Africa/Juba  2:06:24 -  LMT 1931

    Why? And do we need the backzone entry at all?

    Michael Deckers.

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