Bob, I too thought of the Dia Compe hanger that Peter White mentioned. But I suspect that you'd prefer something more period or brand correct for your Herlow, yes?
You mentioned that it has a fastback, allen bolt arrangement. I presume then that one stay is countersunk to accept the allen head and the other stay is threaded (6mm?) for the bolt to thread into.
The only centerpull hanger that I can recall for fastback stays is like the one you have on the Cinelli you were riding on Saturday which has fastback stays and is equipped with Universal centerpull brakes. That one mounts under the seat bolt head which isn't an allen head nor is it countersunk as I recall.
So I don't have an answer for you, but my guess is that it was equipped with something of the same nature as your Cinelli.
Bruce Schrader San Francisco, CA
===== "Not all those that wander are lost." -J.R.R. Tolkien (1892-1973)