The following is my understanding of how the serial numbers get out of sequence.
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The frames are marked with the serial number before they are sent out for finial finish [anodize]. The frames are inspected upon their return to the Factory. Any frames that do not have an acceptable finish, are pulled out and returned to the Anodize vendor, making those frames out of sequence with the next batch.
For example, my Pup is numbered R 600, and I received it months ago.. The gun you received today could be marked with a lower number...
A few years back, I spent a few months in the Remington Firearms Factory in Illion, NY.. I had a lot of time on my hands and a nearly free run of the whole factory. I observed the Serial numbers being Stamped on the long guns, it was virtually the last thing done, just before being boxed and sent to the Shipping / staging area.