I found this at peace tree poultry
"Welbars were created by Mr Humphreys of Eastwrey, Devon who thought of the idea in 1940. He contacted Professor Punnet (Breeder of the cream legbar) for advice. Following the legbar process, chicks from the initial welsummer x barred rock were hatched in 1941. Cockerels from this mating were then bred with welsummer hens to produce gold and silver welbars simultaneously. The breed was recognised by the Poultry Club of Great Britain in 1948."
It seems to imply that the cross with the a Barred Rock can be used to create BOTH the gold and silver forms of the Welbar.