1957 Senator "A".
1957 Senator "A". While the metal expansion band is listed in the text for variants A, B, C, and D, it is not necessary to distinguish between the four, as the case design is different in each variant, and we have no ads in this period which suggest the same case is offered on leather or other mount.
Here's the four variants from 1955 Bulova Matt ads on site.
Senator A but needs the band changed to a flex.