All about Gruen

[Fred]

Possibly several at work—sure I’ve never found all. But US1498221 I think best applies. Able to mass produce intricate stamp designs on a thin layer of gold with base metal outside support, form it to a caseback shape, remove the outside support , add the base metal “reinforcement” inside & seal it—and you have a gold filled case.

[Mr. Bieser]

Thanks! I think you may have nailed it.