I have a 69 DT1 trail that a merchant marine brought to the U.S. from Japan. Can anyone tell me the difference between the DT1, DT1S & DT1B?

Welcome but i'm no help sorry. I had a "69 white tank [think it was an old stock "69 i bought in "70]. My Boss at the time bought a gold tank one in december "68. [first lot to arrive in the country from what i remember] I'm talking NewZealand not US. Someone clued up will put us right. To me they were just a DT1. Cheers. ps still have one [ugly one] but built up from my junk, RT1 frame from about "71 & piston port inlet DT1 motor. Goes extremely well. Worked in the local Yamy shop in those really fun times but maybe i'm just old & reminiscing haha.

VIN number is the best place to start...  and probably cross off plain "DT1"...  all 250 enduro models through 1973 had a DT1 VIN prefix for the most part.

A "plain" DT1 could arguably reference a first-year 1968 model but generally those are now called DT1A...  Full VIN will tell you the year. B and S were available in 1969. 

There's info in the Tech Library manuals and several threads about the "S" model vs.the "B" model.

If it is a bike from the Japanese Domestic Market (JDM), then it will likely have no engine number, the frame number might not be covered by the VIN tables (these mainly cover US models) and will have fitments different to the US bikes.

Lots of photos would be great, and   from the UK.

I would suggest you go to CMSNLs web site and look at the parts diagrams to help as well. Some descriptions might leave you wondering if you don't.
Very early ones had aluminium front mudguards easily identified by the rivets holding the stay attachment brackets having three each in them.
Square swinging arm is pre 70. Early (68) ones also had a bracket that bolted on under the tank . Possibly a bright zinc coated airbox too.
All that's provided it's not been messed with.
The back light is different design on 68 & 69 bikes too.
Also JDM bikes from 68 & 69,dependent on which province they came from, had a light mounted on the front mudguard. Horrible!