I thought S19000 was the last serial number known for a while but since then a couple of slightly higher higher numbers in the 19000 range have come to light. John Atkins mentioned one in his recent multi part series on the Webley Senior in Airgunner and another collector told me about another pistol he had come across, which is also in the S19000 range.
I doubt we'll find many much higher than that though.
John