MarkT. I saw your post. Note, the LEFT bearings ( 00012, 30517, the one that was originally installed), and the RIGHT bearing (20501) "appear" to have the same special radius at least visually.   In any case, I have the 30517 in the LEFT side and 20501 in the RIGHT, so we shall find out soon.  Also noted, the partzilla catalog indicated the 30517 in later DT models. One question though. 


All of the bearings, but the 30517 spin freely out of the box.  The 30517 is a bit more resistant.  Do you think that is a concern or maybe it will just loosen up on run?  
 

The 6305 bearing you said was "not seated" in your pictures goes in the left case does not look to be the correct bearing.  Someone may have replaced it at some point in the past.  It does not have the X1 designation that NTN used for the special radius that allows the bearing to seat properly. 

I'm writing this below post #200 & somewhere earlier i said i can't see the reasonably thick washer on 'your mainshaft' on the left hand end.
I see you now show it fitted in the pic just above where i'm writing under post #200, But actually i now see it doesn't show it in the parts drawing pic in post #200.
So now i'm feeling "even more stupid" than usual.
My thought is it goes between #10 & #11 on the end of the shaft so i don't know nuffink anymore lol.
If you look at the earlier design that i'm more familiar with it shows it in link below.
I guess it doesn't do anything useful unless the clutch nut comes undone--but that'd be the least of your worries if that happened.
link to earlier design below. I keep editing so might read all messed up.
www.partzilla.com/catalog/yamaha/motorcy...973/rt3/transmission
I'm referring to #11 in my RT3 link above.

I'll just add [while watching coronation street on tv haha] that if there's room for the washer 'i've been woffling on about' i'd run it anyway.
For you to have it there i guess it must've had one there before.
If there's room for it as i said!!.
With cases mated up you still need running main shaft end clearance so go careful on a practice case mating.

Mark, I'm confident going forward w/current setup. I would not doubt someone was in the case previously.

RT325, I'm quite sure you are correct on the shim, As I disassembled the engine, I positioned parts as I removed them in a line AND took snap shots. In the picture, the shim is laying directly next to the shaft.  I'm going to try and install the shaft by itself w/the case halves as a test.

Sorry to keep adding & might be just me, but shim shown above looks quite thick!!.
So i'd call that one a ''washer' & look far too thick to be the 'thin shim' that goes against that fixed gear which goes up to the bearing from the inside when all assembled.
That said--the thick-ish washer i'm talking about 'on earlier motors i'm familiar with'' goes the 'other end' of that shaft 'left end', against the needle bearing 'on earlier design 250-360 motors'.
But to confuse it all the parts pic doesn't show it on your "74 model.
Washer you show looks too big ID for the left side end anyway.
So is 'that' one to do with the clutch spacing, on clutch side of the bearing in the case. Maybe goes first on against the bearing in the case
It just looks thick in the photo not wafer thin to go against the fixed gear.
Thin washer i think is on 0.5mm thick.
In the link below i think washer you show is #18 & clutch spacer sits against it with #17 sitting up on the spacer.
www.partzilla.com/catalog/yamaha/motorcycle/1974/dt360a/clutch

The "formed" washer looks sort of like a bearing shield to me.

Umm, yes i never thought of that.
Thought it looked thick with a bit of a taper on the inside like shap corner taken off.
So a shield out of the trans mainshaft bearing as think theyre shielded on the clutch side of that case bearing.
I think you're right.

Pic of OP's bearing on previous page shows no shield & partzilla doesn't show one either. Double row bearing in "74. So i give up haha. I don't know anything anymore.
www.partzilla.com/product/yamaha/93304-2...94cbf9643c279adea53a
I'm a century behind. The older 360's ha a Z bearing which in single shielded.
I've gotta stop comparing models & making a mess of posts.
Part: BEARING (B6205Z SPECIAL) RT2--DT1mx might've ended up on but same horse-- 'trans'.

The good news.  If you are a century behind, then you could look at is as having 100 years experience!    

I may have found an unsplit 360 to look over.