To me, that "steam tractor tool box cover" explanation sounds like a theory rather than a fact. I've seen steam tractors and can't imagine a little 8" x 4" toolbox holding much that would be useful for one of those monsters! And the actual number on the plate is 52, with vertical bars before and after,
not 1521.
Back in the day, when many Australian farms/stations were first established, major providers of necessary equipment for building, fencing, digging, etc, offered pre-crated selections of useful tools and implements via catalogues. So instead of a buyer having to determine which particular items and how many of each and which sizes, they could just pick from the numbered assortments on offer for each type of task and so get most of the necessaries more easily. This is an American example of the same type of thing:
The Aussie crates were well made to survive the rigours of the outback and were intended to provide permanent tool storage, with a printed list of the items within, on the inside of the lid. My guess is that the farmer's metal plate was a tag/label for one such crate.