As always, great input.
No, there are no markings on the shell itself. My guess at a date of manufacture is based on the fact that it was found in the rubble left by the Begbie's explosion.
As I understand it, Begbie's Engineering Works was not involved in the production of ammunition until the foundry was commandeered by the Transvaal authorities at the beginning of the ABW. After the Ultimatum, Thomas Begbie had shut up the works, laid off all staff, and fled south to Cape Town, where he was employed as chief engineer to the Steam Road Transport Company until the end of the war.
So the shell was either cast at the foundry after it had been commandeered, or it was an earlier example taken there for some unknown purpose. MC Heunis wonders whether they had intended to convert the old shells they had in stock (see above).
MC Heunis has also sent me photographs of two further examples of this shell, one with the applied lead jacket sill in place: