I googled this item and got a few references on German language websites.  When I translated them it said it was a plaster hook for concrete.  I still do not know exactly what it is or how it is used.  You can buy new ones on Amazon that are very similar.  This one is marked DRGM, so I think it is pre-war Germany made.

My files are too big too post.  You will have to go to Ebay to look at the photos; sorry.   I have not been able to post many whatits since I got a full time job.  I have put the link for the listing below.

https://www.ebay.com/itm/Vintage-Bertool-Betonhook-DRGM-Plaster-Hook-for-Concrete/402412198692?hash=item5db1a2db24:g:aQAAAOSwCXlfUvVx

I think that is used as a Hold Down, similar to a Workbench Holdfast. Drive into concrete until the outside edge ot he curved piece makes contact with the piece being held. For forms, or, attaching screeds.