16600 is a great tool watch and the last SD model before Rolex changed to the ceramic bezel version. I still wear my SSD on a regular basis. Actually, the 16660 or triple 6 sea dweller was the bridge between the vintage 1665 and the modern versions. All great watches with a lot of history. Enjoy!
Nice build J... actually this isn't a fantasy, Rolex did build these for the Japanese market. Can't remember exactly when however, I believe they called it the "Summer Sub".
I agree with Preacher, let it run in for a couple of weeks, this will help the movement "settle" in. If it continues to run fast, you can attempt to regulate the movement yourself or find a rep friendly watchsmith that could perform this. Have added a link below to a previous thread addressing regulation of different types of movements.