Hi Roy
When I worked at the Wembley 6 Days in the 70's as second mechanic to Peter Post , Patrick Sercu, Pfenniger and Sphan, (just a little name dropping), Duralca rims were very popular with the continental six day riders. They were indeed of Swiss manufacture and had a very wide cross section affording good support to the tyre. The well of the rim had shallow grooves and did not need "scoring" to take the shellac base, as of course all tubs were shellaced on.
Mike Mullett
Reading UK