Some had beaters and some didn't. At some point in time Electrolux stopped fitting beater bars.
The beaters simply slipped into place instead of one row of brushes (although I suppose you could have had two rows of beaters if you wanted to, not that it would have cleaned much). Given that changing the brushes in the roller was a well-promoted task, and as the brushes usually came in sets of two, I am sure many owners dispensed with the beaters. I have seen these types of cleaner of all ages with or without beaters. I think Electrolux stopped using beaters when they changed to the Twin Turbo style of cleaner.
The 506 is a very rare machine. It replaced the 504 around 1980, but was then replaced very quickly by the almost identical 560.