It dawns on me that there would be nothing to lose in asking the seller if the missing parts could possibly be in an attic or garage. If they're clearing an estate, that would just be junk to them. I googled "Singer parlor cabinet" and got lots of hits, some with open doors showing a wide treadle that filled the space below. The drive wheel and belt were enclosed in the area behind the doors on the right, and there appears to be an additional door on the right side panel to give better access to that area. I don't know if the one pictured with that door open is the same model as yours.