Ah, that explains my confusion. ```productDefinitionTemplateNumber``` doesn't exist in GRIB1. I see there's an [existing load rule that uses this] (https://github.com/SciTools/iris/blob/master/lib/iris/fileformats/grib/load_rules.py#L365) without checking the edition, which seems a good reason to provide it here, but it might be worth considering removing it as the api will return 0 for GRIB1. We could then create a ticket to check the edition in that rule - or re-write the whole lot in functionally decomposed loveliness. Your call, it works as it is.