1. My 1st sighting of Bullhead style was on a Breitling [Pupitre model], it wasn't for another couple years that I saw the Seiko 6138 "Bullhead" style ... in Brown color scheme it is called "Cha Uma" [Brown Horse] and in the Black color scheme it is the "Kuro Uma" [Black Horse] by the Japanese.
2. About the same time, I learned of the Citizen cal. 8110 Fly-back chronograph movement and it also had a few Bullhead (or "Easter Rabbit") case styles. In some ways, the Citizen is superior to the Seiko 6139 & 6138 as it also has the Fly-back functionality (like the Seiko 701x family of Fly-back chronograph). The 8100 (up to 30 Minute accumulators, like Seiko 6139 & 7018) & 8110 (up to 12 Hrs like Seiko 6138 & 7016) are both 23 jewels & run at higher beat-rates than the equivalent Seiko calibres. The main "complaint" about the Citizen Bullheads (and some other models) vs. Seiko tends to be the casing, not the movement. Some collectors/wearers think the Citizens are "too small" (they do tend to be more compact than the "big iron" 6138s from Seiko. In addition, certain case-styles of the Citizen don't seem to hold up as well as the typical Seiko chronograph (more delicate details of outer bezels & inserts tend to wear more easily than most Seiko designs) & some of the case styles are plated material (and of course, plating wears faster than all Steel cases).
More info about these at Sweephand's site:http://sweep-hand.org/
However, especially if you don't care about having a giant hunk of iron on your wrist, the upside is that you get a great movement & usually can get the Citizen chronographs at lower price than the equivalent Seiko models. However, this may be changing...
3. And yes, the accumulators do reside in "sub-dials"