There are a few examples of 21 vs 17 jewels movements; 7019 vs 7009, 6309 VS 6306, etc.

In theory, the extra-four jewels makes the movement last longer, with less wear and have better precision. In practice? Haven't noticed much difference between them.

Many 21-jeweled movements back then tended to be JDM (6306 Turtle, for example which had hacking as well).

I'd just buy the watch in the best condition, regardless if it's 17 or 21 jewels.

 

The 21 jewel versions were sold in Asia because the extra 4 jewels serve a purpose (as opposed to other watches where extra jewel count is for adornment and not function).

You probably know but the US imposed extra taxes and duties on watches over 17 jewels so manufacturers down-jeweled to 17 jewels to be price competitive in the USA.

SpeedTimer 6139 variants are all 21 jewel JDM releases.

 

I’ll do my best here to complement coffee and grammar. So, the issue isn’t jewel count and performance as much as the JDM version vs. other releases for the export (from Japan-centric) pov. I’m currently focused on collecting the weird JDM variants of the 6139-7xxx and -8xxx models. Do the jewel count isn’t something I’m tracking, more like the many dial variants within this range.

Now when I’ve had 6139s serviced I always have the main spring arbor ports jeweled because there is a performance and longevity gain in doing it. So all of my 6139s that I have serviced have these. So my 17 jewel watches turn into 19 and the 21 jewel yo 23.

 

I agree, I think out of the 4 extra jewel, maybe 2 are useful, the 3rd wheel and the diafix but the others are not so much.
Arbor port jewel is must have if you plan on wearing the 6139 for long time, but alas none of the 61xx have it save for the high beat Grand Seiko and 6159,

 

That’s the thing ... If two of the “extra” jewels were on the mainspring arbor ports, it would be a big deal. But, alas …