This is the Rikshird mark, it was awarded to members of the Rikshird for unknown criteria. While the criteria I have not been able to figure out, it is believed that this badge was awarded to only about 20% of active Hirdsmen (1300 out of 5000). The RIkshird badge was gold, with the recipient's number on the rear...either engraved or stamped, such as this one. It is usually believes that the badge was made and number engraved prior to being awarded, but that doesn't explain the stamped badges, as the numbers would have had to be stamped prior to the enamel being applied. One theory is the stamped pieces were replacements, or duplicates for wear, etc. The gold finish on these stamped ones, made by David Anderson, tend to rub off, as you can see on this badge.
It is also believed that these may have been the early ones, and then recipients could submit a certain amount of sterling to have a higher quality one produced, but not everyone was keen on melting down heirlooms.
Sadly, no surviving roster has been found to identify recipients of these badges by the number.
The Undhird (youth organization) also had such badges, with silver for teenagers and bronze for preteens. Unnumbered originals, likely later war unissued examples, do exist
Many unanswered questions remain about this badge, but it is a very rare badge to find!
Badge in wear by Thorvald Thronsen in his Rikshird uniform...
Badge in wear by a Norwegian volunteer in the SS...