you could check & see if any 21 day fares are offered on your route in the future....i rarely see them.....

also, you could post the itin & perhaps get some observations....

i don't know of any way to get the info except maybe calling the carrier, but doubt that it would work.....

also, low fares could sell out on 21 day as well as 14,7 & 3.....happens all the time...