I killed a bear in 1985 with the 17-6 and I still shoot them. A great arrow! It is a combo of fiberglass with the dark streaks being carbon.
ID is .281 so a 2016 (.2805) should fit and a 2014 (.2845)just a tad oversize so it could be sanded down to fit. Do not force a insert into a glass shaft, if it doesn't split immediately it will later from stress. The one piece insert could be better as it is a bit longer. Clean the inside of the glass and epoxy the insert. MAke sure the end is cut square.
The insert you have are not plastic, they are probably nylon.