F2 Bin Capacity Question

Got the combine to start shelling corn well.  Thank you to all who helped with my last post.  I still have a few bugs in the system, but overall I am pleased.  I finished a field today, and have been doing paperwork tonight for yield reporting.  I looked in the Manual for the F2, and it shows a capacity of 120bu with extensions.  I was curious if mine has extensions, or if it is standard capacity.  Looking online, I see pics with different toppers above what I have.  Mine has two fold up flaps on the front and rear of the upper box, that are held with large rods across the bin.  Some pics have larger boxes going up where I have the flaps.  

In 1978, a shelled bushel of corn weighed 56.67 lbs at 15.5% moisture. Today, it's measured at 56 lbs at 15.0%. I think gleaner must have rated its grain tank capacity based on shelled corn with a 56.67 pound per bushel test weight? Reason I say that, is because today I never get close to gleaner's spec capacity on corn or soybeans. But soybeans have a 60 pound per bushel test weight. So gleaner's 1978 120 corn bushels at 56.67 lb test weight corn equals 6800 lbs. This figures reduces to 6720 lbs at 15.0%. Theses volume weights would only equal 113-112 bu of soybeans. To complicate it further, different harvested grain moistures play an additional weight role.

DanD,

yup 120 , like all mfgs, is the 'theoretical' capacity. It is obtainable
buy hand filling every kernel.... 110-112 is about the 'practical' capacity. "cab rain" occurs about 10 bu before theoretical capacity is obtained...

The capacity rating is based on wheat not corn and I believe all combine manufactures in the US base their ratings on wheat.