The IB tractors before 1946 did not necessarily have the C channel frame, it was optional. If your tractor's S/N falls before the IB dedicated range which started at IB1001, it would have a S/N interspersed with regular B production serial numbers, as would the orchard B. If your B has a foot throttle, a parking handbrake and a straight, non-adjustable, solid steel (not tubular) front axle, it is probably an IB. It might still be an orchard model, but based on the info I gleaned from Guy Fays book and Terry Dean's data book, this is the conclusion I came to. Of course, you might also have a regular B that someone has modified to try to make it look like an IB or an orchard B or even to try to make a few more bucks by deceiving an unsuspecting customer.
Let us know how this comes out. You've piqued my curiosity.