its hard to tell without being able to see what model it is,i cant read the decals.i believe the gt2500 was an anniversary model.it had an 11horse aluminum briggs but i dont think it had the plastic fender pan.
It's a 1981 GT2500 with the Briggs 11 horse engine. They were a 25th anniversary edition tractor but really the only think unique about them is the special silver decal. When new, it came with a one ounce silver bar as a keepsake...
The rear fender on that tractor has been replaced with the goofy plastic version from the c-XX5 version.
The tractor is fundamentally a standard C-series (C-XX1 type) with an 11 horse Briggs in place of a more typical Kohler. Basically a C-111.
They aren't particularly rare, but they aren't the most common model going either. A lot of collectors like them just because they were an anniversary model, but an equal number of guys don't care for the Briggs powerplant.
The engine was lighter duty than the tractor and many have been replaced with something else thru the years. That tractor looks original except for the fender.