Only two modifications to 4006A

I believe that during the 12 year production run of Bell-matics there was only two modifications made to the movement;

1. The alarm spring was shortened after about the first year of production.
2. The dial had a solid spacer fixed to the back and this was changed to a separate plastic spacer later in the production run (about 1975 ?), i think that this was possible due to a design error in the first instance as you come across so many dials with the solid spacer were the feet have broken off.