I thought the 7801 used the deeper splined aluminum body that Shimano released to stop anyone using anything other then the matching Dura Ace cassette - called an improvement to the free hub design allowing alloy free hubs! I may be wrong but you should do a search to find out, as there was for sure one of the 7800 series wheel sets they did this too which kind of limits the wheels appeal
I thought the 7801 used the deeper splined aluminum body that Shimano released to stop anyone using anything other then the matching Dura Ace cassette - called an improvement to the free hub design allowing alloy free hubs! I may be wrong but you should do a search to find out, as there was for sure one of the 7800 series wheel sets they did this too which kind of limits the wheels appeal