"I took the 230-grain shell to be an explosive shell." -- Why?

British rifle ammunition for the 450/400 3 1/4" cartridge was made primarily (and perhaps exclusively) by Kynoch and Eley.
Their early 20th Century catalogs make no mention of an "explosive shell" nor does Fleming in "British Sporting Rifle Cartridges."

Can anyone cite a source for explosive bullets in British commercial metallic sporting rifle ammunition?

Explosive bullets are historically associated with Sir Samuel Baker and George Fosbery. In Baker's case, his bullets were being
launched from a 4-bore barrel. A detailed study of explosive rifle bullets used in the Civil War can be found in the journal of
the American Society of Arms Collectors. They were used to shoot at observation balloons in WWI and Germany experimented
with the B-Patron in WWII. If they were ever practical - as bullet size decreased, they became impractical.

Pantaman - suggested topic for your next investigation. Weight of the priming compound in the No.40 Berdan primer.