My MM is from '51. chassis nr 122061, and has the first type without a nut.
Because I have a spare got with the car, or better, 2 different, I also have the newer one.
When you put the newer one in it, the mounting holes for the wiper motor to the body are not on the right place.
Also the holes are a little bit too small for the newer type, the thread is damaging the paint when put this in.
When Mike also knows his chassis number(s), we can find out when the newer one was used.
I only know between January and September 1951, a lot is changed.
I only could see, on the pictures from my previous 1956 Series2 with center clock, the nuts were used.
From cars between 1951 and 1956 I did not found any picture from this parts.
I think the newer are mounted with the nut on the box, the older needs the bracket, the holes for this bracket are under the rubber from the front screen.[frame]

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On this picture the holes are clear to see, because of the base coat on the dash.