(24 Jul 2016, 8:34 pm)R852 PRG wrote I can vaguely remember the X21 being increased to every 30 minutes while the 21 went to a lower frequency of every 15 minutes. Might be wrong, but that's my recollection of things.
I can remember a frequency increase on the X21 too. I think it was the time when the X20 was withdrawn so the X21 was increased to every 30 minutes as opposed to being hourly.