The first 10-speed Dura-Ace 7800 series chain was the
CN-7800, but for some reason
it kept breaking in the Southern Hemisphere (to be precise, Australia), and
about half a year after its release it was changed to CN-7801.
The ST-7800 also became ST-7801 fairly quickly,
and while there is no ST-7901, there is an ST-9001,
so people who have been riding road bikes for a long time
definitely tend not to buy the earliest lots of Dura-Ace.
The CN-7800 had both ends of the chain as outer links when shipped
so that no matter who cut it, a fresh outer link would remain.
With the CN-7801, both ends are an outer link and an inner link.
But nowadays the CN-7801 is also discontinued, and as a replacement part for the 7800 series
the Altegra-grade 7800 series
CN-6600 is what you're supposed to use.
But now the box even has Dura-Ace written on it,
so this CN-6600 is different from the old days and is now "treated as Dura-Ace",
and to put it dramatically, the specifications have changed.
That specification is "two quick links are included".
Only Dura-Ace chains come with two, and all other grades come with one—this has been the tradition since quick links began.
The CN-6600 from back when the CN-7801 existed definitely came with one quick link.
So if you spot old CN-6600 sitting in a shop's stockroom,
are you losing out by one quick link?
Well, from that point on, chains have likely gotten so much more expensive
that if you found the old CN-6600 selling at its original price,
you'd actually come out ahead. Though it almost certainly doesn't exist anymore...
CN-7800, but for some reason
it kept breaking in the Southern Hemisphere (to be precise, Australia), and
about half a year after its release it was changed to CN-7801.
and while there is no ST-7901, there is an ST-9001,
so people who have been riding road bikes for a long time
definitely tend not to buy the earliest lots of Dura-Ace.
The CN-7800 had both ends of the chain as outer links when shipped
so that no matter who cut it, a fresh outer link would remain.
With the CN-7801, both ends are an outer link and an inner link.
But nowadays the CN-7801 is also discontinued, and as a replacement part for the 7800 series
the Altegra-grade 7800 series
CN-6600 is what you're supposed to use.
But now the box even has Dura-Ace written on it,
so this CN-6600 is different from the old days and is now "treated as Dura-Ace",
and to put it dramatically, the specifications have changed.
That specification is "two quick links are included".
Only Dura-Ace chains come with two, and all other grades come with one—this has been the tradition since quick links began.
The CN-6600 from back when the CN-7801 existed definitely came with one quick link.
So if you spot old CN-6600 sitting in a shop's stockroom,
are you losing out by one quick link?
Well, from that point on, chains have likely gotten so much more expensive
that if you found the old CN-6600 selling at its original price,
you'd actually come out ahead. Though it almost certainly doesn't exist anymore...