Hard to be sure, since the mounting device/pins has been lost from this piece's reverse, but I would guess that this was made for the grip of an HJ Field Knife. As I recall it, the two-pronged mounting device on those pieces was secured to the reverse of the plate by a similar round base.
The smaller HJ diamonds were in the early years sold for wear with civil, as was the larger form.
This was at the moment the round HJ badge was to be worn with headgear.
They already were offered as "Amtliches Abzeichen" when one could show a membership-card (nur gegen Mitgliedsausweis).
Information from a price-list from about 1933.
Later they were used for other purposes also (see Ron's badges).
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