Larry, there's no telling without pictures, what you have. Originally, D-15 ag tractors would have a PD hi-lo, on the RS, with a PTO lever, on the left, right behind the clutch. PTO is semi-live, with this set-up. You can snap the PD into neutral, and the PTO stays running, while the tractor coasts, or stops. To put the PTO into gear, you have to step on the clutch.
Now here's where complications arise, They also made the yellow line (construction) D-15, and it's later counterparts, the I-60, and I-600. These yellow tractors were available with a shuttle (F-R) clutch, on the left side of the tranny. Also they made later models 615, and the RL451 loader, on the D-15 base chassis. The RL451, was a reversed loader, with the same Shuttle clutch, only I think it used the foot pedal, you describe, as your tractor has. The main complication is, that there are several models of tractor, that can be assembled like a giant tinkertoy, as the components all readily bolt right up. With enough engineers, a tractor can end up looking far different, than it did when it left the factory. Look in Swinford's construction book, if you want the full story.