@pybonacci That makes sense. Thanks.
(I had sussed the accumulation from the grib message attributes, which are effectively the info in the idx, but I hadn’t worked out the pattern. )
I am left intrigued by the logic of this particular strategy of information production. Given these are hourly forecast fields, it would make sense for “the last one hour” in each, or the “last n hours”, but a variable number out to each six hour cycle makes little sense to me But there will be a reason ...