It's very hard to find the reference for this watch as the movement still using "Seikosha" before Seiko.
Here what I found from:
The logo from 1962 as the “athlete dolphin
The reverse side cover of 20 stone waterproof models, in the metal band it seems that the “dolphin mark” is for the first time adopted.
Because (it is the matic first waterproof side product, the proper?)
After that until it moves to reverse side cover hallmark “SEIKO” in around 1967, the dolphin mark had been attached to the reverse side cover of the SEIKO matic waterproof product.
The same “dolphin mark” is seen even in the “sport matic”, but this with like equality design, seems that is not the variation of dolphin design.
Reverse side cover picture
The seiko matic 20 stone
It is thought as the first dolphin mark equipped product of the SEIKO matic. When observing at that the dolphin mark has entered into also the clasp part of the original metal band, there was “small impression”
Seikomatic, Seikosha movement, 20jewels, automatic, keeping the good time, unpolished crystal and case.
Head only, dial has 360' mark due to the previous owner over pushed the hands.
New aftermarket leather.
Asking price: IDR sold,-