On the 500 series the gaps for edge-cleaning came later. The 170 was in production for a very long time so it's possible that the gaps were added at a later date. The model numbers on Electrolux cleaners often had a letter after it; A, B, C, etc, and the further down the alphabet they went, the newer the cleaner was & denoted any that the design had been amended.
As to where the changes took place, well, that could be very obvious or very subtle. For instance the Z87 cylinder had two types of motor, the first being used for a short period, the second being used right until the end. Later on the plastic covering on the cleaner body was changed from stippled to linear, and the lettering moved from top to sides. Another change saw a different type of hose. Each style came with a new letter after the model number.