Trim levels. 170's were 4 door or 2 door sedans only. They had very little chrome trim and many were /6's with 3 speed manual transmissions. 270's were a little more flashy and could be ordered as a hardtop. The GT's were top of the line and had a lot of trim, carpet, bucket seats etc.
Generally, what were the differences between the 170 and 270 in the early a-body Darts? Are there ways at a quick glance to tell the difference between the two (if there are not any 170/270 emblems present).