That's the measurement I have been finding also, so I called FA on the phone to verify length a guess what he said he was very very wrong and they do have one to send me at 15.5". I did ask him if he was absolutely sure it was 15.5" and he confirmed that he measured it himself and it was 15.5". He said it will ship Monday of next week.
Extended length national match gas tube for use with barrels with extended gas systems.
Length: 17.170" (I have 2 of these if anyone is interested)