Old Town 145251 is an 18 foot CS (common sense or middle) grade square end paddling canoe, which was finished May to August of 1946 with ash decks/thwarts/seats, outside stems, half-ribs, row locks and rowing last, and was originally dark green. Image of the scan of this record is attached below-- click on it to get a larger image.
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/wcha/ to learn more about the WCHA and http://www.wcha.org/join.php to join.
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.
I know they do come up once in a while in these serial number searches. You could do a search here, using "square stern" in the "search" function on the right above. The only way to know for certain how many were made is to count them, which is an impossible task given the fact of over 200,000 records!
Kathy is correct that it would not be easy to get an exact total but the Old Town database has a sample which can be used to make estimates as shown at http://www.wcha.org/catalogs/old-town/models.html for several other models. The sample size is much smaller for the 18 foot long square ended paddling canoes so this estimate will be less accurate. It appears that about 144 of these were built. They show up in the sample database between 1931 and 1970 which works out to about four per year. Have fun with yours,