If you visit this page of the website with the best archive of Herse bicycle photographs, and if you look closely at how the handlebar bag attaches to the stem, you can see an original. There's also an illustration below that shows how cleverly it works.
If you are looking for a modern version, you should visit this page on Compass Bicycles to see their decaleurs, which are Japanese-made and beautiful quality.
My bicycle was missing this crowning touch for the front of the bike. I purchased the Compass Bicycle version and it worked great. But then I saw the photo I linked to above, of the genuine Herse decaleur and I couldn't stop thinking about it and decided I had to have one like it on my bike. A couple of months later I have one.
To explain how it attaches to the bicycle, It mounts beneath the custom Herse stem on its twin bolts. You can just see the holes in the second photo. They're covered by masking tape in the first photo.