That was a standard marching size that found it's way into being used as both floor tom, and later on, as a mounted tom size. I think Ludwig and Slingerland both offered it as the floor tom size for their less expensive offerings (Club Dates and Stage Bands) although eventually both switched to 14" toms for those. Later on (70's) there were many sets that included that size as a mounted tom. Of course there was nothing stopping anyone from ordering a tom that size with legs installed and many a 12x15 marching tenor or snare drum has wound up being converted to floor tom use. They do work pretty well as floor toms.

Thanks a lot for the info

+1 with what K.O. said! Here's a link showing the 12x15 as a floor tom in the Stage Band outfit, which makes perfect sense with the 9x13 rack tom!

http://www.vintagedrumguide.com/images/catalogs/slingerland/1964_slingerland_9.jpg

Mark