Bolt Size Variant In Same Year and Model

[Bingster]

On my '47 De Soto S-11, the bolt thread for the the floor board hold downs is 1/4-20.  It looks to be a larger washer on Thrashingcow's S-11 floor board.  I believe his was a Canadian made De Soto.  I admit the 1/4-20 washers do not seem big enough. They do cover the holes, and the threads are definitely 1/4-20 in the floor, but I am somewhat puzzled by the difference in size.  Anybody out there who has an S-11, could you tell me the bolt size of your floor bolts?