I found a parts list for the 101
http://www.tandtrepair.com/Schematics/101-1.pdf
On page 8:
66525 is the slide plate complete with spring and screws. Singer did not mark any pieces with 66525. The complete part would have come in a box, bag or envelope printed with 66525. The plate is 66524 and is always marked. The screws are not likely to be marked. The spring may or may not be marked.
So, 66525 is the repro, and 66524 is the original. We've seen this before with bobbin cases. The repros are marked with the part number of the complete set, and the original Singer parts have the individual part numbers marked on one or more of the pieces (usually the biggest piece, in this case the plate). But if you find 66524 on ebay, be sure to get a view of the back and check that the spring is on it. On second thought, the spring protrudes a bit on the sides so you can see it from the front, but it's still a good idea to get a view of the back to check for rust or breaks.