Hi Tim,
There were a lot of different K&B 40 engines so let's quickly eliminate some. The rear rotor RC pylon racing engines were screamers so eliminate them.  Hopefully, what you have is is the 70's era RC sport engine. However, we are not done identifying yet. There was a series 69 engine that had a wedge shape combustion chamber and mild timing that gave a good stunt run for one gentleman I met. I bought a used series 70 engine. This engine had a hemi head combustion chamber. This head was identified with a small "P" stamped on the front side of the head. This engine had mild timing and gave a a good stunt run with only the addition of a venturi. It has a dykes ring, it is cross scavenged and it has the small front end. Our groups K&B's were the 1st low pitch higher rpm engine runs I remember. Props were Rev-Up 12x5's cut to 11 5/8" x 5 set at 10,000rpm +/-. The series 70 changed to squish band head, bigger bypass, different timing when it became a popular sport pylon engine. It was later named series 71 and even more changes were added. These can be made to work but the early series 70 worked, as is, for me.
I reccomend getting a ring from Mr. Bowman. Flying buddy, Stan Powell liked what he saw and spent a lot of time & money refining the breed, especially getting a good ring fit and Les McDonald won world and national championships using the the engine that resulted.                        Pat Robinson

I've flown the 4011 engine a lot and really like them.  They are very powerful.  This is the one that takes a 1/2 in OD venturi, has a larger front ball bearing and no rollpin through the drive washer.  I ran APC 11 x 5 props.  My engines ran excellent upright and inverted, but I was not happy with them side mounted.  Side mounted a Tower 40 worked much better for me.  I flew a Humongous with an upright mounted example and an OD 720 square inch flying wing with one inverted. 

All K & B 40's have two ball bearings.  I have engines like you say, and it is a really small front bearing. It may also hae a smaller hole for the venturi.   All are Dykes ring except some later ABC versions, so far as I know.  The engine you have will run fine.