New to me as well. For example India has UTC+05:30: https://en.wikipedia.org/wiki/List_of_time_zones_by_country
Librem 5 BroadMobi modem reports wrong (?) manufacturer/model info -- bug in firmware? Where to find documentation for the modem?
A…M -ex.J and N…Y, plus Z are the military time zones. They are completely inadequate to represent all the world’s timezones (as they don’t allow half-hours, never mind about quarter-hours).
Of the military time zones, only Z is valid in ISO 8601, I believe. However a system processing a date in ISO 8601 format may have been implemented to be more tolerant.
Other systems still, like RFC 2822 (mail header format), explicitly allow military time zones, but only as an “obsolete” specification e.g. for compatibility with very old stored email.