Hi Again Melb,
I've just had a look at my identically coloured Kirby powered Solid Deck 17 inch unit which has the following numbers on the ID plate;
45-05387
Yous is;
45-20211
This is a difference of 14,824 machines if the numbers were issued in sequence, but to this day we still don't have confirmation of how their numbering system worked, but I guess it could be possible that they made around 15,000 machines per year which equates to 60 machines per day, sounds doable I guess taking into account that they were all different models and configurations and as such the number after mine could've been a standard type of slasher and the one after that was a gang mower, so on so on. As the numbers were never stamped by machine until much later it would've been easy to just make up the ID tags on the bench in sequence to suit each machine as it was finished and packaged up for dispatch. One day I'm sure we'll find out, but if I'm to be correct then it makes my mower to be older than yours.
Hopefully DeeJay might join in to this conversation when he gets back with some fresh ideas.
Cheers,
BB