British shotshells cases length

Which year did the British shotshells manufacturers (Eley and Kynoch)change their 12, 16 and 20 shotshells case lengths from 2 1/2 to the common 2 9/16 ?
jp

It may be a dumb question but have you asked Eley?

I don’t think Eley can answer me because it happened a long time ago between 1878 and 1898.
But it is a good suggestion anyway.

jp

Most of these companies have (or had) an archive and a company historian of some sort to field enquiries.
Trouble is people tend to retire and not get replaced.