Certain concrete processing components are FET exempt

We have just begun selling concrete dispensers that carry ingredients for making concrete to construction sites. At the construction site, the dispenser's conveyor and mixing systems combine the ingredients to make concrete. These dispensers are mounted on trailers having a GVW in excess of 26,000 pounds. A customer told me these dispenser units were not subject to the excise tax under IRC Section 4051 because a specific exemption from this tax exists for articles that mix concrete. Is he correct that we do not have to pay any tax on the concrete dispensing units?