An about 1929 manual price page showing part DN Bearing Right Angle and part DU 1/4 in. Diam. Axle 8 1/2 in. These parts came in various sets 7 to B in years around 1929. The question arises as to what models required these parts.

My first Erector set manual admonished me to start with small models and proceed to larger models. The manuals themselves attest to this good advice. The model plans are somewhat lacking and vague in many places. The plans show minimal part labels and leave the builder to guess as to what parts are required. This is doubly true of the parts in question. I have not been able to find any model picture in a manual that shows both these parts labeled. But a model requiring one part will need the other part too.

Picture in a 7 1/2 manual showing part DN, the bearing. The line stops short of the part which is at the bottom of part AC.

Picture in a B manual showing part DU, the axle. The part label is near the label AC at the right in the picture.

Pictures of the model that I built with the 7 1/2 set. The 7 1/2 set has tires on the wheels, so I used the wheels from my B set to match the plans for the model in the manual. The axles are parts DU and the bearings for the axles are parts DN, attached to parts AC.

This is the underside of the second model. This was built with a B set, but my B set wheels had no tires, so I used the wheels with tires from the 7 1/2 set.

Bill Bean's Erector set inventories show that the parts DN and DU never were included in the number 7 Erector set. But this  page from a 7 manual shows the part DN used for the Box Coaster.