Pete, as nobody else has tackled your question, I thought I'd give my two pennuth, for what it's worth.
Whilst we all know that 1934, some of 1935 and 1944 to 1946 pennies, were mint toned, actually finding out why is not that easy. The only thing I can find is what Freeman wrote at page 17 of "The Bronze Coinage of Great Britain", 1985 edition.
That's the nearest I can find, although you could always contact the Royal Mint for further information, or ask to see the Royal Mint report for 1934. It's not 100% clear. I also had a look in Peck, but apart from an acknowledgement that they were mint toned, I can't see anything else in there.