The Australian numbered stickers were first introduced in May 1978 on the Series 1. This also happened to be the first year the X1/9 was sold into Australia. 

Each dealer was given a batch of stickers to place on the car at the dealership, or the buyer would get the sticker and put it on the car themselves. This means there is no relationship from sticker to VIN or spares number. The only connection is the batch of stickers the dealership was given by Fiat. 

The stickers continued to be placed on cars till late 1981 and only by a few dealers that had leftover stickers. 

The very first X1/9 sold was a Green series 1 bought in May 1978. The owner was asked what number he wanted. He did not pick 001 but rather 007 for obvious reasons. I interviewed original owners and dealers of the time to get this information.

As I have an almost complete list of VIN's, I think it's fair to assign your car from this list.  However, to preserve those with stickers already. You will notice that I have placed a TAKEN in front of some numbers. This is to avoid two cars having the same number and to preserve your original numbers assignment. This list is the X1/9 Australia re-assignment list, not the original list. This way of assignment probably was how Fiat intended but could not execute at the time. Please also note that the numbering will keep improving as other cars are found. So you may need to reallocate your number in the future.