Fender doesn't sell replacement necks as far as I know...sorry.

Fender do sell original replacement neck
they have only 2 option for Strat
(1) 21 frets, c shape profile, rosewood (like a '62 )
(2) 21 frets, v shape profile, maple (like a '57)

[quote="thejakeludwig"]Can I buy a lefty strat neck with the same serial # as my right handed neck?
all serial number is individual and will not be the same
so no luck getting a left handed neck with same serial number

each Fender neck has it's very own serial #, and comes on a Fender guitar. A Fender licensed neck is identical to the neck that comes on a Fender guitar. why do you want to replace your neck if I may ask?

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you can save the world with your guitar one love song at a time it's just better, more fun, easier with a fender solid body electric guitar or electric bass guitar.

i had just read the first post again and i realised i mistaken something
you are looking for a left handed neck on a right handed strat, right?
it is possible to get from ebay
i mistaken your meaning of serial number as the guitar serial number
if the size is same
it should fit
sorry i do not know anything about the neck marking serial (number)
hope you get your new setup fixed soon

normally when people says guitar serial number
it refers to the guitar serial number on the headstock

What you seek is absolutely not available now or at any point in the past from Fender. At one time Fender sold replacement necks and bodies, but never with custom ordered serial numbers. You would have to buy a left handed neck of the same vintage and replace the nut and live with the different serial. They had a model setup like this called the Dick Dale Stratocaster. You could look for one of those.