A lock washer on a postage stamp engines needle?

I would agree, and would not have questioned it were it not a perfect fit.  It does provide a fine tune of the needle with each slight turn resulting in a "click." You can see the bitter end of the spring in the above picture.  But why would the PO do it?  Seems quite unnecessary.  

Just one of the more interesting things I encounter while playing with these pre-owned models.

Bob, the previous owner may have replaced the stock spring with one having larger dia and without the washer the spring would slip over the shoulder of the NV?

I also pull a small washer on the NV to prevent the sealing fuel line from slipping over the shoulder. The fuel line, if cut to proper length, will double as a spring too. (And I have also added a home made screen to this Killer Bee backplate)