The Old Town with serial number 137772 is a 17 foot long HW model canoe and the one with 187772 is an 18 foot long fiberglass canoe. Please take another look at your serial number and let me know which digit is the least clear. It should be stamped on the inside bow stem (although it is often under the mast step) and on the side of the transom. Feel free to reply here if you have any other questions.
Benson