I'm not sure why you would assume anything in that video is 'legal', but there are transferable registered HK sear packs. There was one in an MP5 at NFA Day 2010.Around 6:25.
I swear that looks exactly like an M16A1
I know those go for about $20-$26k
However... I've never seen a full auto mp5 in my entire like.... that was legal to a citizen.