All, apologies for checking this thread a few days late, but I can add my data point that pretty strongly supports the theories expressed so far here. I got my near-NOS proof silver 6139-6000 from the original owner along with two other watches - a 6119-6020 sports diver and a 6139-6000 proof gold dial - who reported buying all three in Hong Kong in January of 1970. The silver's dial is 6030T, the day wheel is English/Chinese, and the bracelet is a Stelux chiclet. Even more on point, the silver came with the Thong Sia sales paperwork, filled out to list the serial number that matches the watch. So I have no doubt it was sold there. Pics:
Also, I agree there was definitely no JDM silver dial version - I say this having devoted an unhealthy amount of time to tracking down every possible dial color, dial type (resist/proof/blank), and target market (speedtimer or not) example of 6139-600X and 6139-603X. I'm still missing an export silver resist and an export silver blank, but having tracked down the rarest of the rare (the Sunrise, the Forward, the 603X resist Italian export, and the 603X blank French export), I'm sure at this point I would've seen at least one silver JDM if it existed. All that said, if anyone's ever interested in selling/trading for a silver resist or blank, please reach out!