Thanks, Helene, for explaining your reasoning.
Derek has covered the basic point about canes possibly being made in one period but still being used in later periods. I know, also, that some of the so-called "Salvador canes" (see the various literature) can be found in some of Paul Ysart's work likely to have been made after Salvador and the other sons went off and formed the "Ysart Brothers" company. So it's even more complex than the basic "Ysart Brothers - Vasart Ltd - Strathearn" chain of events.
If the overall evidence of a piece points to it having been possibly made at the Ysart Brothers company, then there is just as much chance that Salvador made it (or finished it) than anyone else. It might be true that, at that company, items with certain canes were only ever made by Salvador, but that does not also imply that Salvador did not make items with "ordinary" canes.
The only way I know to be sure that a particluar item was not made by Salvador Ysart is with a Shortwave UV test. After Salvadaor died, and Vsart Ltd was formed, the glass batch changed and this can be picked up with the UV test.