Your description of the DC66 brush bar part explained the situation already. There are two versions of the DC66 Vacuum, one needs short tabs and one needs long. I dunno why Dyson didn't further differentiate those two models into like.. DC66A and DC66B or something, I imagine half the people who order this part get the wrong one not realizing there are indeed more than one version of the DC66.
Can you send us a picture of the current brushroll you have?