According to the 1973 Schwinn catalog, a P15 would have come with 27" x 1-1/4" high pressure clincher tires standard. the brand of rim isn't specified but Weinmann seemed to be very common in this era. Two tubular options were offered. Wheels and tires aren't specified for the P10, but I would think they are the same.
http://www.schwinnbikeforum.com/SLDB...73xlite02b.jpg