FAA approval
Not exactly sure what makes it the major mod, may well be the autopilot coupling. There are a couple of other issues which have 337's for conformity. It took a while to find the correct procedure to follow, I'm not questioning the amounts I have been quoted, I have already agreed the price, its time its actually taking after originally been told it would be a two week process. I am also now informed that the same outcome can be obtained by having an approved faa avionics shop inspect and certify the installation.