The Old Town canoe with serial number 58613 is an 18 foot long, CS grade, HW (Heavy Water) model with red Western cedar planking, open spruce gunwales, 20 inch birch decks, birch trim, a keel, and outside stems. It was built between March and June, 1920. The original exterior paint color was dark green. It shipped on May 1st, 1920 to Buffalo, New York. A scan of 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/catalogs/old-town/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.html to join.
I have an 18 foot long Guide's model from 1931 that should have the thwarts and seats spaced similarly to yours. The middle thwart is obviously in the middle of the canoe. The bow thwart is 34 inches ahead of the middle thwart. The stern thwart is 34.5 inches back from the middle thwart. These were probably supposed to be symetrical so you may want to adjust yours to match each other. The bolt holes for the stern side of the bow seat are 39 inches from the middle thwart. The bolt holes on the bow side of the stern seat are 61 inches from the middle thwart. All of these distances are from the centers of the bolt holes measured along the top of the inside rail.
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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