Perhaps we should lose the 3 BILL cap altogether.

In grib, a negative int simply has the top bit set(!).
So, according to the grib api, a negative grib int of -3 is interpreted as 231 + 3 (i.e 2 BILL + 3).

To complicate antagonise matters, anyone who sends a negative forecast period to the grib api will have it interpreted as a large positive number close to 232 (4 BILL), as the grib api will receive a two's compliment number.

Thus, we start the workaround at 2 BILL (really negatives near 0) and cut it off at 3 BILL (really -1 BILL).
Anything above that could either be a two's compliment, and therefore not a grib int, or a really big negative grib int. We can't tell, so we currently ignore it. Happy to lose the 3 BILL cap if requested.