Just a simple task: a second edition of Rob's 1996 book 'A to Z of Australian-made Motorcycles'.
We've put a few thousand hours into it so far. The 'problem' is that there is more info available now - for example digitised newspapers and collated Victorian registration details - so that even a 'quick' revision of an existing entry can open a can of worms.
Using "Utility" as an example, the first edition says they were sold between 1931 and 1940, yet there is NO Utility motorcycle advertising before 1934, even though Utility Motor-Cycles Pty Ltd was formed in 1931. Thus my interest in the first registration date of a Utility. In the scheme of things it doesn't really matter whether the correct date is 1931 or 1934, but in principle the date can be known, and it would be better to use the correct date...
There are more esoteric Victorian examples. The Clyst was built by A. E. Head of Yarram, country Victoria, c1914, and was written up in the James Flood Book of Motorcycling in Australia in 1982, with a couple of fabulous photos of Dr Rutter's Clyst, powered by a JAP twin, with Truffault fork and Armstrong hub gear. The story (repeated in the 1st edition of A to Z) was that this was the only Clyst, but I know there was at least one other, a 4 hp JAP single built in 1913 offered for re-sale in 1914. How many more were there? Maybe the Victorian records can say how many Clysts were registered? But it doesn't matter too much - "a small number" would be an adequate description for the book.
Or the Cottman Colt. (You can tell I'm doing the "C"s.) The 1st edition says 1935 - 1941, I'd say 1939-1941, the rego records say??? And numbers? An informed observer from the time says "several hundred" but what if the rego record says 40 or 400? If it's a number that could be had at a glance, I'd be interested to know it.
As I mentioned above, the incomplete South Australian rego records are useless for solving the "how many" question. If there were 3 ZZZs listed, you can't say "at least 3" because it may be the same bike registered 3 times with a new number each time. Or if there was only one YYY listed, you can't say "only one" because there could have been 10 others where the new owners kept their old rego numbers to put on their new bike.
Cheers
Leon