In 1966, there were relatively few assault weapons on the market that were convertible to full automatic, and far less awareness of how to do so. Now the instructions for converting a weapon are freely available on the internet, along with kits costing just a few hundred dollars that either help with the modifications or essentially act as crude conversion mechanisms in and of themselves.
There weren't as many AK-47s around in 1966 as there are now, and only a few AR-15s that could be converted, because it was a relatively new weapon...