7881 (and 7882) were discontinued and consolidated with part numbers 2412 (and 2413) following a factory acquisition. The 2412 & 2413 were previously 'CDT' part numbers and had more market recognition so they were kept. (Short & sweet, the fact that the former numbers were discontinued is irrelevant as mentioned above.)
The cheapest from China are being found to often have copper-clad-aluminum conductors and counterfeit UL labels and to be non-compliant in multiple aspects, The UL website has multiple posts warning about such findings. Stick with reputable known manufacturers.
Cat 6 standard is 250MHz and systems operate at that frequency and lower. Yes, the 2412 is 550MHz . Most manufacturers carry premium model numbers that are quality checked up to higher frequencies than 250MHz because such tells us the cable is even better than the minimum required, is more durable (has more headroom -- to absorb some abuse and still perform to spec.) and/or otherwise just provides a margin-of-error / safety factor. SOME (but not all) 500MHz / 550MHz cables may have extra tape and/or "spline" inside that may make them more of a pain to put the connector onto. For residential use - a 250MHz rated cable will be absolutely fine if it's from a reputable manufacture. "Headroom" 500MHz / 550MHz cables are more for data-center use.
Cat 6A is a wholly different cable standard with higher bandwidth (lower cross-talk / lower noise rejection requirements) than Cat 6 -- and a higher frequency (625MHz) quality check required not optional. Belden spec 10GX12 is one of their model numbers for Cat 6A (has such a "spline as mentioned above / is a greater pain to connectorize.and very likely requires a special custom connector) -- and Belden has premium model numbers above 10GX12 as well, some with shields and therefore requiring connectors that are designed for shielded Category Cable.
Source (in part) Public Cable 1.0 -- a book on Amazon -- has simplified but detailed cable specifications for Category cable and more than 80 other low-voltage communication cable styles from the simplest constructions suitable for speaker cable -- on up; and lists up to four (all domestic) manufacturers and their equivalent part numbers. A great resource for anyone specifying communication cables / audio, video, speaker, or control cable styles; or for anyone looking to cross one manufacturer's part number to others.