I hate to be a "wet blanket" here, but I'm very dubious about the meaning (and possibly the authenticity) of this letter.

The Transportation Security Administration imposed very restrictive limits on what an individual airline may allow as carry-on luggage in the "post 9/11" tightening of regs. A specific limitation in that "crackdown" was that no passenger could carry on more than one item of luggage.

It is doubtful that the author of this letter has any authority to require airlines to accept instruments as carry-on items, although the decision would permit them to do so if they choose to. Individual airlines, one would presume, may still set their own limits on the size and kind of items they choose to permit.

In any case where you expect to need "special consideration," it would still be advisable that you communicate directly with the airline involved, and inquire specifically with respect to the flight and flight dates scheduled as to whether the special treatment you require will be made available on that date and on that flight by that airline.

Since this "decision" changes a specific published ruling, I am surprised that there is no citation of the specific rule that is changed in the letter. (On the other hand, good writing skills do not necessarily get one an appointment in the Federal Beaurocracy.)

Before putting too much faith in this letter, though, I would want to obtain a copy from an "verifiable source," such as direct from the FAA - Security regulations site. I've seen a lot of "more authentic looking fakes" than this letter - but then I've worked with government agents quite a lot, and we made lots of jokes about them (in private, of course). They probably did the same about us.

John