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Regarding the way that the lower part of the extractor is ground away, here are a couple of thoughts.

First, is that these rifles use a staggered cartridge positioning in the magazine, meaning that the cartridges do not slide straight up the center of the bolt face as they are being fed into the chamber.
Each cartridge will come up out of the mag off center and at an angle due to their staggered magazine position.

All Mausers with box mags similarly feed, but the extractors on them are not so ground. This confused me and the g-smith as to why CZ would do this.

Second, is that while the '98 style bolt diameter remains the same for different chamberings, the diameter of the cartridges and the cartridge heads do change.
The different diameter cartridges can lead to different width magazine boxes.
This can effect how far off center the cartridge is when it needs to slide up the bolt face and under the extractor claw during controlled feeding.

Third, is that CZ's are mass produced and would require the easiest method to obtain some degree of reliability with feeding as the come from the factory.
I think this could be the reason for that relief at the bottom of the extractor claw.
It could allow the cartridges to feed better from the right side of the magazine, allowing the rim to more easily find its way under the extractor claw as the cartridge is released from the magazine.

Your second and third points are probably near the mark, tho still, other makers have not taken this approach. Post '93, "cheap" Ruger MKII's for example, seem to provide pretty good CRF, at least the ones I've checked mostly do. The CZ gunsmith did tell me that the extractor for the 6.5x55 is the same part as the extractor for .308/.30-06, i.e., "8x57mm Basic Case".

From what I was told some time ago by CZ, tho, CZ550's are NOT supposed to be "snap over" 98's and extractors for the 550 series normally need modification in order to reliably snap over. Thus, we have an extractor grind that may allow the case to pop forward of its grip, but an extractor that isn't really designed to snap over in this event.

It seems that the best normal practice might be to have the extractor altered {if need be} in order to snap over. The gun would then possibly not have CRF, but would at least not allow rim-forward jams. The gun would then function in effect like a SAKO AV; snap-over, non-CRF, but with the controlled-ejection feature common to the Ruger MKII and Win 70 {a feature I have personally always thought was more important than CRF anyhow.}

I myself may look for other extractors that might be "drop-in's" for the CZ550 or able to be altered to fit the CZ action. I have no idea if there are any.

Keep in mind, all this applies especially to the small-caliber guns. The gunsmith told me that the larger calibers {not sure which all are involved here} normally are CRF. This seems to make sense in light of Hoppdoc's pic of his .505 bolt.

Rule of thumb seems to still apply: Check ALL rifles for reliability, especially DG rifles.