You've got a very nice tractor there!
I think it should be a 1972 model tractor. I believe the 15 hp cast iron Briggs engines were only built for approximately one and a half years in 1971-72.
Oops. Totally missed that it was a 15hp instead of 16. You're right, they used a 14hp in '71, 15hp in '72, and then 16hp for '73 and up.