There's a wonderfull book out there called "Six Days of Madness" describing, mostly, pre-WWII 6-day races.
One of the things you eventually notice, paging through the photos, is that many of fork crowns on the bikes were Single-Plate like the one for sale now on ebay (130230623747) at:
http://cgi.ebay.com.au/Vintage-Barb-Front-Fork-Track-Bike-1930s_W0QQitemZ130 230623747QQihZ003QQcategoryZ42319QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
When did these go out of fashion and why? Were they ever used on road bikes?
Amir Avitzur
Ramat-Gan, Israel