Hello Blumbly

I think I can answer your question about the Model 4901589 & 4908706.
Why do they have an extra digit? Answer: They are not model numbers.

What you have are mower number plates.
In your photo the plate actually says just that. It's a mower serial number.

On some machines, SB included an extra plate with the Model number:-
[Note how the mower no. and the model number begin the same]
[Linked Image]

So, your machine model number(s) should be interpreted as per
my list in the ODK History Record.

Hope this helps.
-----------------------------
JACK