Odds that Schwinnn/Panasonic frame is JIS headset?
 

1976 Super Le Tour 12.2. Frame built by Panasonic. What do you suppose the odds are that the headset is JIS rather than ISO, due to Panasonic being the manufacturer? I know, I know, the only way to find out for sure is to remove the cups and measure with calipers. But I would prefer to buy my new headset and do the removal and new installation all in one quick process, rather than tearing apart my daily rider to do the measurement, putting it back together, and then tearing it down again to install the new headset a week or two later.

I cannot find any specification documentation online to conclusively prove this to be JIS or ISO. The only thing people seem to obsess over on this bike and other Panasonic-framed Schwinns is that Schwinn insisted on installing a fork with a thicker-walled steerer tube, thus destroying the ability to easily access aftermarket stems without resorting to turning the stem down on a lathe (been there, done that to a Nitto stem).

So, gamblers, lay the odds on JIS versus ISO, and I will roll the dice and buy a new headset.

I've learned over the years via home remodeling/repairs, when in doubt by both, and return what you don't use. It sames a repeat run or waiting for the part you should have gotten. Saves a bunch of time.

Changing a JIS frame/fork to ISO spec is a trivial operation, should it be needed.

I can't vouch for the '76 Super Le Tour, but I measured and replaced the headset on a '78 Le Tour III and it had a combination of ISO/JIS parts. The stem was 0.833 (21.1mm), the crown race (fork) was 26.4mm (ISO), while the cups were 30.0mm (JIS).

On a similar note the '73-'76 World Voyageur/Voyageur II had a similar ISO/JIS combo except for the stem, which was 22.2mm.

I'd be interested to know if you could cram 30.2mm (ISO) cups in place of the 30.0mm (JIS) originals, I didn't try that.

"I'd be interested to know if you could cram 30.2mm (ISO) cups in place of the 30.0mm (JIS) originals, I didn't try that"

Sheldon Brown seemed to think so. See headsets on the Harris Cyclery site about JIS:

JIS One-Inch Threaded Headsets for Bicycles from Harris Cyclery

Never tried it though. Would be interested to hear from someone who has.

I've bought mixed JIS 30.0 / ISO 26.4 headsets, new, off of ebay, made specifically for this type of application.