You may need to take another look at those serial numbers and the information at
http://forums.wcha.org/showthread.php?791 may help. None of the build records for serial numbers 103115, 103775, 103175, and 103715 show an 18 foot long canoe or one with sponsons. The canoes with serial numbers in the 703xxx range are too new to have sponsons. It may also help if you can post some pictures here showning the serial numbers from each end. The image below shows another serial number from this period with a seven and some ones for comparison.
My guess is that you have the Old Town canoe with serial number 108115. This is an 18 foot long, CS (Common Sense or middle) grade, HW (heavy water) model with red western cedar planking, open spruce gunwales, a keel, outside stems, a floor rack, sponsons, and a bangplate along the full length fo the keel. It was built between November, 1930 and June, 1932. The original exterior paint was similar to the one shown at
http://www.wcha.org/catalogs/old-town/designs/design37.gif which was known as the design number 37. It shipped on June 13th, 1932 to Russell, Mass. A scan showing this build record can be found by following the link at the attached thumbnail image below.
This scan and several hundred thousand others were created with substantial grants from the Wooden Canoe Heritage Association (WCHA) and others. A description of the project to preserve these records is available at
http://www.wcha.org/ot_records/ if you want more details. I hope that you will join or renew your membership to the WCHA so that services like this can continue. See
http://www.wcha.org/about-the-wcha/ to learn more about the WCHA and
http://store.wcha.org/WCHA-New-Membership.html to renew.
It is also possible that you could have another number or manufacturer if this description doesn't match your canoe. Feel free to reply here if you have any other questions,
Benson