SX 440 is a prototype for the 1975 SST F/A series and is very rare. The hoods and tunnels are slightly different though. I also think the seats and a few other parts are different as well. They are supposed to have an engine that is similar to a 74' GPX or Thunderjet.
if it is infact the early production prototype for the 75 sst F/A it is quite rare. the bumper should have an orange stripe on it not blue also the belt guard will be orange. the fuel tank and shroud are completely different than the 75 sst and will be the hardest part to get if yours is damaged. the hood is also different it doesnt have the big scoop in the hood. NOS parts are non existant for this sled as it was a prototype and no spare parts were likely built. so unless the sled is in remarkably good original condition with all the correct parts intact and inplace its hard to say how much its worth becuase to restore it you would likely have to buy up several other early prototype sleds just for parts. it would seem to me though that you got the sled from some old guy and are looking to make a killing on reselling it.