The full Suprebe derailleur group was standard on this model in 1978, hence the model name. I've got literature that shows it equipped with Superbe derailleurs as late as 1981, but in 1982 Cyclone II were spec'd. I'm wondering if the derailleur is an aftermarket item to accommodate that freewheel? Not all the catalogs indicate the freewheel size, but based on the specs and pics, the 1978, 1981 and 1982 models were 21T, maybe 24T at the most. Maybe the orignal owner could not handle the gearing and changed the freewheel and derailleur? This was fairly common. Also, the saddle appears to have been switched for something a bit more cushy, would would fall in line with the other suspected changes.
Based on the 1978 literature, yours would appear to be sligthly newer. The 1978 literature does not show brazed-on shift levers, recessed nut brakes, a integral slopping fork crown, a chromed chain stay or a decal on the headtube. It looks like it's time to start checking component date codes. What's the serial number?