Final instalment. I should add that the left hand switch block looks to be early DT1 but you may be able to advise whether Yamaha continued to used it on later models. Should I post a photo of the switch block?

the switch bracket on the frame was used in 1968-1970 on both the 250 and 360.

Here is the identification for your speedometer/tachometer sets:

Thanks Devin - love your work, truly amazing how helpful this site is.
Are you able to identify the headlight? Possibly common across a number of models too I'm guessing.

The headlight bucket looks like a 73 DT3. I think your core bike is a complete 1973 Yamaha DT3 fitted onto a 1969 DT1 frame, and they included a bunch of spares and misc parts.

Thanks Devin - your conclusion makes sense to me about the bike, as it stands, being what we call a "bitsa" meaning bits a two or more bikes cobbled together to make one machine.
What I am dying to know is whether I can restore and original "matching numbers" DT1 using the frame DT1-35809 and the loose motor that came with the bike DT1-13794.
Parts I appear to need already are the correct oil tank, exhaust and small tacho.

So you want to make a 68 DT1 with a 69 DT1 Frame/68 DT1 engine? The 68 DT1 has many 1 year only parts, and the parts are so rare, almost none left in the world, people like Yamahabob have started to have replicated parts made. If you are just trying to make a rider that looks like a clone from a distance that is possible. But buying NOS parts to make a 68DT1 is impossible.

What ever you do, please document it.

So it sounds like the 68 DT1 engine I have isn't the one that come out of that frame originally - is there a register of frame & engine numbers or a formula that accurately identifies original matching frames/engines?? For example, another very collectable Jap road bike, the Z1 Kawasaki has a formula whereby the numbers have to be within a narrow range of each other.
My plan is take it back to as original as possible within a managable budget - so I might stick with that exhaust and headlight for example. But I am very keen on the smaller sized tacho - it just looks so right for the period. And the 250 enduro oil tank also provides the look I'm after.
Anything thoughts on the seat? The photo I provided shows some clear differences between the bases of the one currently on the bike and the one that presumably came off the bike.
Rest assured I will document the restoration with lots of pictures and progress reports.

Here is the vin charts:

www.yamaha-enduros.com/index.php/workshop/vin-numbers

Thanks Devin but that chart only shows engine numbers. I guess I'll just to be satisfied that on my current research the DT1 motor DT1-13794 was not the original motor in 1969 DT1B frame DT1-35809. I've had the bike for over 10 years and alas had believed it was the original motor from that frame. And that the tank and seat shown in the photos were part of a previous incarnation of the bike...