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Maybe you got a rebuilt lense with a replacement nameplate, which if it's available by itself, might lack a stamped in and white paint filled serial.
In the computer world, you can replace any part on a server, including the case, and some stickers, but they won't come with serial number. IBM used phoney looking hand written labels in situations like this.
I have the same lense as you, and the only visible place the serial exists is on the nameplate, on the registration card and the box.