3849850 and 3849851 are the metal ring and the boot, which they call a retainer and a seal, respectively. I'm going off of the '65 assembly manual, page 258. On this page they mention a 'seat separator and trim plate' which we know to be an SS console, and that's what the picture depicts, installation of these items on a '65 SS. There is no call for this color coded plastic ring under the 'console' or 'seat separator and trim plate'.
The assembly manual does not reference this plastic part. However, my '65 Nova wagon came with one, turquoise, and color coded to the carpet. I don't even think it was stapled to the carpet, just set in the cut out and it never popped out or never went anywhere. But again, that was a 400 series with an M-20, so it would have the ring.
My '64 100 series sedan with factory 4-speed had only the rubber mat, a black boot and a black ring to hold down the boot, screwed in over both the boot and the rubber mat, bench seat shifter. I think that's the way they came for 100 series.