I’m doing some work on it to prepare for the upcoming riding season.  Last year I noticed the clutch was slipping under load, so am in the process of tearing that apart.

I couldn't get the retaining nut loose on the adjusting screw down on the left side engine case to see if that would help (lever was fully adjusted, but still slips).  Is there a trick to getting that loose?  Decided to pull apart the clutch and check specs on parts:

 I’ve found some inconsistencies with the manuals I have (3 different ones) and the bike itself and was looking for any guidance you might have.

First and most concerning is the plates and friction pads.  My bike has 6 friction pads and plates with a solid plate (see photo) behind everything.  All the manuals (and the friction plate replacement kit) have 7 pads and not the solid plate (which I don’t see on any schematics)

Another question is the correct clutch spring specs.  2 manuals (vintage yamaha service manual) say 36.4mm and one (Chiltons)shows that for the DT250 its 31.5mm.  both my existing and new springs I bought are more like 32.9mm.  Which is correct?

 Remove the cover that the clutch cable goes in. Grab the push lever assemble with a pair of channel locks on the arm to hold it. Dont grab the round part with the pliers because you will ruin it. then use a socket and ratchet on the lock nut to loosen it. 

You will use 6 friction plates, 7 clutch plates 1 clutch spacer plate(your picture) and a metal clutch plate1. You will start with clutch plate 1.  It's different then the others. Then clutch plate spacer (smooth no teeth plate) then a metal clutch plate, then a friction plate then a metal plate until you end with a metal clutch plate on the outside. Shop Manuals are not always correct. 

Best to replace all friction plates, metal clutch plates and springs to do the job proper. Or at least clean and check all metal clutch plates for warpage. USE yamaha parts if all possible to avoid doing it over or still have clutch slippage. Buy the springs from Yamaha.  

This picture looks correct and was the way my 68 DT1 clutch was when i took it apart and put back together. Notice clutch plate 1 (#4) is different then the 7 metal clutch plates. (#8). The spacer plate ( you called it a solid plate ) is # 5 in the picture. 

 
 

Didn't the 360's use the extra plate and the 250's had the spacer plate? Been a while since I have read that somewhere.

I'll add to this. I have several NOS complete clutch assemblies. They came with 6 friction plates. I do believe the later 360s used a 7th friction plate instead of the spacer.
The springs in some of them measure 30.5mm and and some measure 33.5mm. Could be the 33.5mm were for intended for MX models or 360s. They may have compressed a little after being under tension for 50 years. New springs that are still available from Yamaha  part # 90501-20122 measure 32.5mm and # 90501-20124 measure 34mm.
Friction plate # 5Y1-16331-01 still available from Yamaha has better friction pads than the original. They also have thicker tabs that go between the basket fingers which will reduce "combing" of the fingers.
For a 250 I would use the shorter springs and 6 friction plates. For the 360 longer springs and 7 friction plates.

And check the nylon spiral on the push actuator for cracking. And check the basket fingers for grooves ("combing").

Thanks all.  I've attached a manual page which most closely resembles what I disassembled, and a closeup of the order of the friction pad, spacer and plate in the order they were in.  Also a full pic of the pieces.  Based on the comments and the drawing, am I missing a metal clutch plate? Or should I just put it back together like it was?  I did order new clutch springs from Yamaha, 90501-20122, which I'll use (but existing ones are same length)

NOS clutch assembly:

 

From what i see in your pictures your missing clutch plate 1. #4 in the parts schematic. It's a little different then the other 7 metal clutch plates. Take a good look at your plates. Are all your metal clutch plates the same? clutch plate 1 is a tad thicker and is slightly different. If you see one is different then your missing 1 metal clutch plate. Check out the picture 1971 DT1 250 posted. It goes on 1st then the thick spacer plate goes on next. then the 1st of the 7 clutch plates next. Then a friction plate next just like the picture shows.  

You should have a total of 9 metal plates. 7 are the same, 1 is a thick spacer plate which you have. the last one is slightly thicker then the 7 and goes on 1st. 

Kinda odd your missing a clutch plate though? 

Yep, weird about the missing plate.  6 are the same, 1 thicker one, and one with slightly different tangs on it, which was the 2nd plate in the sequence.  I'll get a whole new set of plates I think.

Yep, weird about the missing plate.  6 are the same, 1 thicker one, and one with slightly different tangs on it, which was the 2nd plate in the sequence.  I'll get a whole new set of plates I think.

Also, is there supposed to be a cushion ring #6 on the diagram?

I leave out the cushion rings when I rebuild a clutch. I don't feel any difference.