A pdf of the score certainly helps in this case. Interesting to note that the bass saxophone/contrabass clarinet part drops on the C a full bar before the tuba part adds the lower notes.
It's also interesting to see the 1st and 2nd trombones drop an octave a measure before that...on the measure that starts with a Bb. One has to wonder why it wouldn't have been better to add the lower octave in the basses in that bar as well.
The bass clarinet drops out after the highest bar and doesn't finish the phrase at all, even though the bassoons and bass sax/contra clarinet continue the passage to the end. That makes no sense to me at all, unless Holst was used to hearing a really loud "honky" bass clarinet that couldn't decrescendo effectively...even that doesn't make sense considering the other woodwinds continue the line on down.
There is no string bass part notated on the original Boosey score, but page 2 makes it obvious that Holst composed one with the "Str. Bass solo" notation (he didn't even bother to notate that passage...you have to understand instinctively that the string bass doubles the low woodwinds there).
But, I suspect, Holst (as a master orchestrator) simply wrote the line and decided "gee, I better not have an entire section of basses up there above the staff...time to add the extra octave."