This piece begins with a section in Bb Major that is concevably divided into a repeated phrase. I feel that the first phrase goes 4 m's and then has an IAC and then has a consequent phrase that goes 7m's and then PAC's. (asymmetrical) This is then basically repeated. So the first section is tonally closed.
Then we have a new theme in FM that is another period of 8m (symmetrical). First 4m and then an IAC and then another 4m and a PAC. We then have a return of the first theme. Given that this is in the dominant of the first theme, we get a rather binary feeling so far from this.
But then we have an additional section different from the theme in F. This is in fm and is composed also of approximately an 8m period. I feel the melody suggests as HC after 4m and then the PAC elides with the return of the original theme at m53.
Each section is tonally closed, so there is a strong additive feeling. And were the repeats not taken into consideration this would be very much a candidate for a 5 part Rondo in my opinion. Therefore the main theme or A section acts as an organizing structure.