Now I'm going to have to do some more research to find out if this is original or modified - so far I've not encountered any documentation that show this style parlor cabinet coming with a flip top. Though if it's been modified, it would be very easy to tell in person.
I would expect the most likely scenario is the lift mechanism broke, so it was removed, the top taken off and a flip top installed in it's place, and the lift button removed and the hole plugged.
Though a skilled woodworker could have replaced some pieces instead of plugging holes, if they really wanted it to look nice. The lady I just got all the treadle stuff from showed me a cabinet (same basic style as this but with the embossed trim) and she admitted she had taken apart three cabinets and used pieces to put together this one that was near perfect. She did an excellent job finishing it too.