My bet is that it is a 3300. Lots of them were listed as Timbermaster or something as well.
Definatly not a 3500 as like you said it has the narrow covers but it also has the narrow opening in the front of the case for the narrow muffler as well.
Here are a couple pictures of my nice 3300. $60 at a flea market.

There nice runners and fine handleing.