Kurt nailed it. One way to make use of those bosses would be to fabricate something similar to the Tanaka decaleur, which appears to thread into the sides of a similarly equipped spacer. However, the attachment point will always act a bit like a hinge, no matter how much it’s tightened, so it might not be the most stable decaleur.
http://store.somafab.com/tadeforth11.html
I’d be more inclined to swap it out for a bespoke spacer decaleur made by Mark G:
https://www.flickr.com/photos/gugie/albums/72157665229628403
Good luck with the new bike! Getting front rack, bag, and decaleur details all reliably and aesthetically sorted out is a character building exercise right up there with interval training and metal fender installation. ;-)
Scott
Atlanta, GA