halfpenny and penny alloy changes?

[Mister T]

I'm surprised that Mr Ts reference (The Bronze Coinage of Great Britain) has British bronze changing in 1923, have to wonder why London gave instructions to the branch mints that they themselves were tardy to follow.

It does seem odd, though to quote the book "although variations in the alloy occurred [between 1860 and 1923], it was officially set in 1923".