Is this receiver unusual?

[cruiserdan]

Here is a 5 digit 11,000 range gas trap receiver with something I have not seen before. All the other low-numbered receivers I have owned/seen have a totally flat op-rod rail. This one has the later type round notch at the rear. The receiver has not had the rails welded for the 7th round. Has anyone else seen an early receiver like this one? Any input would be appreciated.

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[RCS]

I have seen a receiver s/n 12103 that is without the circular cut at the rear and another receiver in the 16xxx serial number range also without the circular cut at the rear of the rail. I have seen rifles in the mid thirty thousand number range with the circular cut at the end of the rail. The early receivers had a darker, smooth finish

[cruiserdan]

Other than being re-parked this receiver has all the other early attributes, including low ribs, except for the circular cut. I have three 4 digit receivers and one 16,000 receiver, all of which are flat.