The Japanese name translates to "watch dish", but I think a better translation is "bowl watch". You may also hear of these chronographs [and their partners, 6139-7100] termed as "helmet" cased.
Here, the much less common 6138 model of the "helmet" or "bowl".
First, the graphite & grey dialed "flavor":
This one has a lot of original case texture, but I don't think you can tell very well from the pic.
Next, the black & blue dialed "flavor":
This one was nicely restored [case] in the US by a friend, so it does NOT represent the original case texture you should look for when getting one of these - but it sure does look nice anyway!
Here's a look @ the original buckle:
I am 99.99% sure these were only offered as "JDM" [Japan Domestic Market] models with the "5 SPORTS Speedtimer" dial variety & the dual English/Japanese day wheel.
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