There is a similar Seiko caliber V157, with the main difference being that the V157 only has a date window at 3:00, and the V158 has a day/date at 3:00. As of this post, there is not a V158B, V158C, and so on. As with all Seiko movements, the trailing letter indicates the evolution or change-log of the movement, starting with A as the first release. The movement in your timepiece may actually be signed with the caliber number V158A.
Some of the topics covered in this caliber listing:
