I think the debate is getting off of Kelsey Tires' point. They're commenting on two types of white letter tires, Polyglas's with the size also in white letters, and those without. Initially they were done without, and changed to "sized" configuration early in the 1970 Model year. This became a big point to '70 Boss 429 show competitors, as all 499 of these cars were built prior to the end of calendar year '69 and were fitted with no-size F-60-15 Polyglas tires originally. Until recently, there were no reproductions of the "no size" Polyglas tires and original, perfect sets of five were selling for upwards of $5K. (Yikes).
Wide tread GT's were a simpler, cheaper tire with similar lettering to the Polyglas. I just found a couple of used original F70-14 Wide Tread GT's. Thanks bkhpah, now I know what they're for
One probably needs to go in the trunk of our red '69 Chevelle L-78/M-22 car....