I poked through the catalogs* and don't see the Guide offered at less than 16 feet. Except for the years 1906-1909, when it was offered at 17', it was 18-20 feet long until 1954, when it began to be offered at 16 feet. Old Town may have built a smaller Guide on special order, I suppose.
The HW was offered at 15 feet until the late '20s. A customer could have ordered a "low end" 15' HW.... and maybe that would appear to be a smaller Guide...?
In general, when someone posts the serial number for a 15' canoe, I expect to see the record for a 50 pounder (trapper/lightweight/featherweight).
Is this a canoe you're looking at, or are you just wondering?