Things i never new about the forks. Were changes both within the steel slider forks ?. We modified our dampers to US trick mods by bronzing over holes & drilling others or oversizing others [too long ago]. Mine still go up & down--or down then up lol.

They used a different damper rod than DT1B , and had a top out spring added as well .
The fork spring preload spacer was different too , but the spring was the same .
 

OK - I just did some comparisons based on the owner's manuals, and maybe the "s" stands for sport or something like that. the DT1B has a low=-mount front fender and a 19" front wheel

 

While the DT1S appears to come with both a 21" front wheel and a High Mount front fender.

 

There may be more things as some people have already said, but it does NOT appear to be the gearing or anything motor wise.

Swoop - What are you referring to as your “DT1S Part Supplement”?

Apologies to everyone for my repeating this for about the 50th time...  The 1971 DT1E Factory Service Manual in the Tech Library has several bulletins in the back that clearly describe many of the differences between the 68 A, 69 B, 69-1/2 S, 70 C, and 71 E models. 

Reply to " Bullnose "
Yamaha produced a Supplementary  Parts List for the DT1S . It basically listed all the items
that Differed from the DT1 B parts manual . a pretty slim tome , all of five pages .
Transmission Gears , Front forks , Fuel Tank , Oil Tank , and Front Wheel .
The Fuel and oil tanks on a quick look , are identical to the B , but in both cases ,
There is a Pearl White option available , so they came in either Candy Orange or Pearl White .
My parts list is from the US market , and was intended to supplement the DT1 B parts list .

Sell - I searched through a stack of 68-69 vintage magazines as well as both the English and Japanese Dealer news from my archives and could find no reference to a DT1C. Will continue the search.

I don’t see any turn signal indicators on that picture of the DT1-S. 

a